Chapter 7. IP Addressing

Addressing is a key function of network layer protocols that enables data communication between hosts, regardless of whether the hosts are on the same network or on different networks. Both Internet Protocol version 4 (IPv4) and Internet Protocol version 6 (IPv6) provide hierarchical addressing for packets that carry data.

Designing, implementing, and managing an effective IP addressing plan ensures that networks can operate effectively and efficiently.

This chapter examines in detail the structure of IP addresses and their application to the construction and testing of IP networks and subnetworks.

Class Activity 7.0.1.2: The Internet of Everything (IoE)

In this activity you will begin to think about not only what will be identified in the IoE world, but how everything will be addressed in the same world!

IPv4 Network Addresses (7.1)

This section provides an in-depth overview of the IPv4 networking protocol that functions as an address for devices connected to a network.

IPv4 Address Structure (7.1.1)

In this section you will learn the essentials of IPv4 addressing.

Binary Notation (7.1.1.1)

To understand the operation of devices on a network, you need to look at addresses and other data the way devices do: in binary notation. Binary notation is a representation of information using only ones (1s) and zeros (0s). Computers communicate using binary data. Binary data can be used to represent many different forms of data. For example, when typing letters on a keyboard, those letters appear on screen in a form that you can read and understand; however, the computer translates each letter to a series of binary digits for storage and transport. To translate those letters, the computer uses American Standard Code for Information Interchange (ASCII; pronounced ask-ee).

Using ASCII, the letter “A” is represented in bit form as 01000001, whereas the lowercase letter “a” is represented in bit form as 01100001. Use the ASCII translator available in the online course to convert ASCII characters to binary.

Activity 7.1.1.1: ASCII Digital Translator

Go to the online course to perform this ASCII translation practice activity.

Although it is not generally necessary for people to concern themselves with binary conversion of letters, it is necessary to understand the use of binary for IP addressing. Each device on a network must be uniquely identified using a binary address. In IPv4 networks, this address is represented using a string of 32 bits (1s and 0s). At the network layer, the packets then include this unique identification information for both the source and destination systems. Therefore, in an IPv4 network, each packet includes a 32-bit source address and a 32-bit destination address in the OSI Layer 3 header.

For most individuals, a string of 32 bits is difficult to interpret and even more difficult to remember. Therefore, we represent IPv4 addresses using dotted decimal format instead of binary. This means that we look at each byte (octet) as a decimal number in the range of 0 to 255. For clarification, 1 byte equals 8 bits equal one octet. To understand how this works you need to have some skill in binary to decimal conversion.

Positional Notation

Learning to convert binary to decimal requires an understanding of the mathematical basis of a numbering system called positional notation. Positional notation means that a digit represents different values depending on the position the digit occupies. In a positional notation system, the number base is called the radix. In the base 10 system (also known as the decimal system), the radix is 10. In the binary system, we use a radix of 2. The terms radix and base can be used interchangeably. More specifically, the value that a digit represents is that value multiplied by the power of the base, or radix, represented by the position the digit occupies. Some examples will help to clarify how this system works.

For the decimal number 192, the value that the 1 represents is 1 * 10^2 (1 times 10 to the power of 2). The 1 is in what we commonly refer to as the “100s” position. Positional notation refers to this position as the base^2 position because the base, or radix, is 10 and the power is 2. The 9 represents 9 * 10^1 (9 times 10 to the power of 1). Positional notation for the decimal number 192 is shown in Figure 7-1.

Figure 7-1 Positional Notation

Using positional notation in the base 10 numbering system, 192 represents

192 = (1 * 10^2) + (9 * 10^1) + (2 * 10^0)

192 = (1 * 100) + (9 * 10) + (2 * 1)

Binary Number System (7.1.1.2)

In IPv4, addresses are 32-bit binary numbers. However, for ease of use by people, binary patterns representing IPv4 addresses are expressed as dotted decimals. This is first accomplished by separating each byte (8 bits) of the 32-bit binary pattern, called an octet, with a dot. It is called an octet because each decimal number represents 1 byte or 8 bits.

The binary address

11000000 10101000 00001010 00001010

is expressed in dotted decimal as

192.168.10.10

Activity 7.1.1.2: 32-Bit Binary Represented as Dotted Decimal

Go to the online course to perform this dotted decimal practice activity. In the interactive graphic, select each button to see how the 32-bit binary address is represented in dotted decimal octets.

But how are the actual decimal equivalents determined?

Binary Numbering System

In the binary numbering system, the radix is 2. Therefore, each position represents increasing powers of 2. In 8-bit binary numbers, the positions represent these quantities:

2^7 2^6 2^5 2^4 2^3 2^2 2^1 2^0

128 64 32 16 8 4 2 1

The base 2 numbering system only has two digits: 0 and 1.

When we interpret a byte as a decimal number, we have the quantity that position represents if the digit is a 1, and we do not have that quantity if the digit is a 0, as shown in Activity 7.1.1.2.

Figure 7-2 illustrates the representation of the decimal number 192 in binary. A 1 in a certain position means we add that value to the total. A 0 means we do not add that value. The binary number 11000000 has a 1 in the 2^7 position (decimal value 128) and a 1 in the 2^6 position (decimal value 64). The remaining bits are all 0, so we do not add the corresponding decimal values. The result of adding 128 + 64 is 192, the decimal equivalent of 11000000.

Figure 7-2 192 in Binary

Here are two more examples:

Example 1: An octet containing all 1s: 11111111

A 1 in each position means that we add the value for that position to the total. All 1s means that the values of every position are included in the total; therefore, the value of all 1s in an octet is 255:

128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255

Example 2: An octet containing all 0s: 00000000

A 0 in each position indicates that the value for that position is not included in the total. A 0 in every position yields a total of 0:

0 + 0 + 0 + 0 + 0 + 0 + 0 + 0 = 0

A different combination of 1s and 0s yields a different decimal value.

Converting a Binary Address to Decimal (7.1.1.3)

Each octet is made up of 8 bits and each bit has a value, either 0 or 1. The four groups of 8 bits have the same set of valid values in the range of 0 to 255 inclusive. The value of each bit placement, from right to left, is 1, 2, 4, 8, 16, 32, 64, and 128.

Determine the value of the octet by adding the values of positions wherever there is a binary 1 present:

If there is a 0 in a position, do not add the value.

If all 8 bits are 0s, 00000000, the value of the octet is 0.

If all 8 bits are 1s, 11111111, the value of the octet is 255 (128 + 64 + 32 + 16 + 8 + 4 + 2 + 1).

If the 8 bits are mixed, the values are added together. For example, the octet 00100111 has a value of 39 (32 + 4 + 2 + 1).

So the value of each of the four octets can range from 0 to a maximum of 255.

Using the 32-bit IPv4 address, 11000000101010000000101000001010, convert the binary representation to dotted decimal using the following steps:

Step 1. Divide the 32 bits into four octets.

Step 2. Convert each octet to decimal.

Step 3. Add a “dot” between each decimal.

Video 7.1.1.3

View this video in the online course for a demonstration on how a binary address is converted to a dotted decimal address.

Activity 7.1.1.4: Binary-to-Decimal Conversion Activity

Go to the online course to perform this binary-to-decimal conversion practice activity. This activity allows you to practice 8-bit binary-to-decimal conversion as much as necessary. You should work with this tool until you are able to do the conversion without error. Convert the binary number shown in the octet to its decimal value.

Converting from Decimal to Binary (7.1.1.5, 7.1.1.6)

In addition to being able to convert binary to decimal, it is also necessary to understand how to convert decimal to binary.

Because we represent IPv4 addresses using dotted decimal format, it is only necessary that we examine the process of converting decimal values of 0 to 255 for each octet in an IPv4 address to the 8-bit binary.

To begin the conversion process, we start by determining if the decimal number is equal to or greater than our largest decimal value represented by the most-significant bit. In the highest position, we determine if the octet number is equal to or greater than 128. If the octet number is smaller than 128, we place a 0 in the bit position for decimal value 128 and move to the bit position for decimal value 64.

If the octet number in the bit position for decimal value 128 is larger than or equal to 128, we place a 1 in the bit position for decimal value 128 and subtract 128 from the octet number being converted. We then compare the remainder of this operation to the next smaller value, 64. We continue this process for all the remaining bit positions. Look at Figures 7-3 through 7-8 to see the process of converting 168 to the binary equivalent of 10101000.

Figure 7-3 Converting 168, Step 1

Figure 7-4 Converting 168, Step 2

Figure 7-5 Converting 168, Step 3

Figure 7-6 Converting 168, Step 4

Figure 7-7 Converting 168, Step 5

Figure 7-8 Converting 168, Step 6

Follow the conversion steps in Figures 7-9 through 7-13 to see how an IP address is converted to binary.

Figure 7-9 Convert 192 to Binary

Figure 7-10 Convert 168 to Binary

Figure 7-11 Convert 10 to Binary

Figure 7-12 Convert 10 to Binary

Figure 7-13 Combine the Converted Octets, Beginning with the First Octet

Activity 7.1.1.7: Decimal-to-Binary Conversion Activity

Go to the online course to perform this decimal-to-binary conversion practice activity. This activity allows you to practice decimal conversion to 8-bit binary values. You should work with this tool until you are able to do the conversions without error. Convert the decimal number shown in the Decimal Value row to its binary bits.

Activity 7.1.1.8: Cisco Binary Game

The Cisco Binary Game provides a fun way to learn binary numbers for networking.

Game link: http://forums.cisco.com/CertCom/game/binary_game_page.htm.

IPv4 Subnet Mask (7.1.2)

An important part of mastering networking is understanding the use of subnet masks. This section provides you with the principles.

Network Portion and Host Portion of an IPv4 Address (7.1.2.1)

Understanding binary notation is important when determining if two hosts are in the same network. Recall that an IP address is a hierarchical address that is made up of two parts: a network portion and a host portion. But when determining the network portion versus the host portion, it is necessary to look not at the decimal value but at the 32-bit stream. Within the 32-bit stream, a portion of the bits makes up the network and a portion of the bits makes up the host.

The bits within the network portion of the address must be identical for all devices that reside in the same network. The bits within the host portion of the address must be unique to identify a specific host within a network. Regardless of whether the decimal numbers between two IPv4 addresses match up, if two hosts have the same bit pattern in the specified network portion of the 32-bit stream, those two hosts will reside in the same network.

But how do hosts know which portion of the 32 bits is network and which is host? That is the job of the subnet mask, as shown in Figure 7-14.

Figure 7-14 IPv4 Address and Subnet Mask

When an IP host is configured, a subnet mask is assigned along with an IP address. Like the IP address, the subnet mask is 32 bits long. The subnet mask signifies which part of the IP address is network and which part is host.

The subnet mask is compared to the IP address from left to right, bit for bit. The 1s in the subnet mask represent the network portion; the 0s represent the host portion. As shown in Figure 7-14, the subnet mask is created by placing a binary 1 in each bit position that represents the network portion and placing a binary 0 in each bit position that represents the host portion. Note that the subnet mask does not actually contain the network or host portion of an IPv4 address, it just tells the computer where to look for these portions in a given IPv4 address.

Similar to IPv4 addresses, the subnet mask is represented in dotted decimal format for ease of use. The subnet mask is configured on a host device, in conjunction with the IPv4 address, and is required so the host can determine which network it belongs to. Figure 7-15 displays the valid subnet masks for an IPv4 octet.

Figure 7-15 Valid Subnet Masks

Examining the Prefix Length (7.1.2.2)

This section looks at the use of the prefix length in IPv4 addressing.

Network Prefixes

The prefix length is another way of expressing the subnet mask. The prefix length is the number of bits set to 1 in the subnet mask. It is written in slash notation, a “/” followed by the number of bits set to 1. For example, if the subnet mask is 255.255.255.0, there are 24 bits set to 1 in the binary version of the subnet mask, so the prefix length is 24 bits, or /24. The prefix and the subnet mask are different ways of representing the same thing: the network portion of an address.

Networks are not always assigned a /24 prefix. Depending on the number of hosts on the network, the prefix assigned may be different. Having a different prefix number changes the host range and broadcast address for each network.

The following figures illustrate different prefixes using the same 10.1.1.0 address. Figure 7-16 illustrates the /24 to /26 prefixes, and Figure 7-17 illustrates the /27 and /28 prefixes.

Figure 7-16 /24 to /26 Prefixes

Figure 7-17 /27 and /28 Prefixes

Notice that the network address could remain the same, but the host range and the broadcast address are different for the different prefix lengths. In the figures, you can see that the number of hosts that can be addressed on the network also changes.

IPv4 Network, Host, and Broadcast Addresses (7.1.2.3)

There are three types of addresses within the address range of each IPv4 network:

Network address

Host address

Broadcast address

Network Address

The network address is a standard way to refer to a network. The subnet mask or the prefix length might also be used when referring to the network address. For example, the network shown in Figure 7-18 could be referred to as the 10.1.1.0 network, the 10.1.1.0 255.255.255.0 network, or the 10.1.1.0/24 network. All hosts in the 10.1.1.0/24 network will have the same network portion bits.

Figure 7-18 10.1.1.0/24 Network

As shown in Figure 7-19, within the IPv4 address range of a network, the first address is reserved for the network address. This address has a 0 for each host bit in the host portion of the address. All hosts within the network share the same network address.

Figure 7-19 Network Portion

Host Address

Every end device requires a unique host address to communicate on the network. In IPv4 addresses, the values between the network address and the broadcast address can be assigned to end devices in a network. As shown in Figure 7-20, this address has any combination of 0 and 1 bits in the host portion of the address but cannot contain all 0 bits or all 1 bits.

Figure 7-20 Host Address

Broadcast Address

The IPv4 broadcast address is a special address for each network that allows communication to all the hosts in that network. To send data to all hosts in a network at once, a host can send a single packet that is addressed to the broadcast address of the network, and each host in the network that receives this packet will process its contents.

The broadcast address uses the highest address in the network range. This is the address in which the bits in the host portion are all 1s. All 1s in an octet in binary form is equal to the number 255 in decimal form. Therefore, as shown in Figure 7-21, for the network 10.1.1.0/24, in which the last octet is used for the host portion, the broadcast address would be 10.1.1.255. This address is also referred to as the directed broadcast. Note that the host portion will not always be an entire octet.

Figure 7-21 Broadcast Address

First Host and Last Host Addresses (7.1.2.4)

To ensure that all hosts within a network are assigned a unique IP address within that network range, it is important to identify the first host address and the last host address. Hosts within a network can be assigned IP addresses within this range.

First Host Address

As shown in Figure 7-22, the host portion of the first host address will contain all 0 bits with a 1 bit for the lowest-order or rightmost bit. This address is always one greater than the network address. In this example the first host address on the 10.1.1.0/24 network is 10.1.1.1. It is common in many addressing schemes to use the first host address for the router or default gateway address.

Figure 7-22 First Host Address

Last Host Address

The host portion of the last host address will contain all 1 bits with a 0 bit for the lowest-order or rightmost bit. This address is always one less than the broadcast address. As shown in Figure 7-23, the last host address on the 10.1.1.0/24 network is 10.1.1.254.

Figure 7-23 Last Host Address

Bitwise AND Operation (7.1.2.5)

When an IPv4 address is assigned to a device, that device uses the subnet mask to determine which network address the device belongs to. As previously mentioned, the network address is the address that represents all the devices on the same network.

When sending network data, the device uses this information to determine whether it can send packets locally or must send the packets to a default gateway for remote delivery. When a host sends a packet, it compares the network portion of its own IP address to the network portion of the destination IP address, based on subnet masks. If the network bits match, both the source and destination host are on the same network and the packet can be delivered locally. If they do not match, the sending host forwards the packet to the default gateway, to be sent on to the other network.

The AND Operation

The AND operation (also referred to as ANDing) is one of three basic binary operations used in digital logic. The other two are OR and NOT. Although all three are used in data networks, AND is used in determining the network address. Therefore, our discussion here will be limited to logical AND. Logical AND is the comparison of two bits that yields the following results:

1 AND 1 = 1 (see Figure 7-24)

Figure 7-24 1 AND 1 = 1

0 AND 1 = 1 (see Figure 7-25)

Figure 7-25 0 AND 1 = 0

0 AND 0 = 1 (see Figure 7-26)

Figure 7-26 0 AND 0 = 0

1 AND 0 = 1 (see Figure 7-27)

Figure 7-27 1 AND 0 = 0

The IPv4 host address is logically ANDed, bit by bit, with its subnet mask to determine the network address to which the host is associated. When this bitwise ANDing between the address and the subnet mask is performed, the result yields the network address.

Importance of ANDing (7.1.2.6)

Any address bit ANDed with a 1 bit value from the subnet mask will yield the original bit value from the address. So, a 0 (from the IPv4 address) AND 1 (from the subnet mask) is 0. A 1 (from the IPv4 address) AND 1 (from the subnet mask) is 1. Consequently, anything ANDed with a 0 yields a 0. These properties of ANDing are used with the subnet mask to “mask” the host bits of an IPv4 address. Each bit of the address is ANDed with the corresponding bit of the subnet mask.

Because all the bits of the subnet mask that represent host bits are 0s, the host portion of the resulting network address becomes all 0s. Recall that an IPv4 address with all 0s in the host portion represents the network address.

Likewise, all the bits of the subnet mask that indicate the network portion are 1s. When each of these 1s is ANDed with the corresponding bit of the address, the resulting bits are identical to the original address bits.

As shown in Figure 7-28, the 1 bits in the subnet mask will result in the network portion of the network address having the same bits as the network portion of the host. The host portion of the network address will result in all 0s.

Figure 7-28 IPv4 Address with Subnet Mask and Network Address

For a given IP address and its subnet, ANDing can be used to determine what subnetwork the address belongs to, as well as what other addresses belong to the same subnet. Remember that if two addresses are in the same network or subnetwork, they are considered to be local to each other and can therefore communicate directly with each other. Addresses that are not in the same network or subnetwork are considered to be remote to each other and must therefore have a Layer 3 device (like a router or Layer 3 switch) between them to communicate.

In network verification/troubleshooting, we often need to determine two hosts are on the same local network. We need to make this determination from the perspective of the network devices. Due to improper configuration, a host may see itself on a network that was not the intended one. This can create an operation that seems erratic unless diagnosed by examining the ANDing processes used by the host.

Lab 7.1.2.7: Using the Windows Calculator with Network Addresses

In this lab you will use the Windows Calculator to convert between numbering systems, convert an IPv4 address and subnet mask into binary, determine the number of hosts in a network using powers of 2, and convert a MAC address and an IPv6 address to binary.

Lab 7.1.2.8: Converting IPv4 Addresses to Binary

In this lab, you will convert IPv4 addresses from dotted decimal to binary and use bitwise ANDing operation to determine network address. Applying network address calculations is also covered in this lab.

Activity 7.1.2.9: ANDing to Determine the Network Address

Go to the online course and use the interactive activity to practice ANDing for network address determination.

IPv4 Unicast, Broadcast, and Multicast (7.1.3)

This section presents the three types of packet transmission. Assigning IP addresses is covered first.

Assigning a Static IPv4 Address to a Host (7.1.3.1)

Let’s discuss identifying a host by assigning a static IP address to the host.

Addresses for User Devices

In most data networks, the largest population of hosts includes the end devices, such as PCs, tablets, smartphones, printers, and IP phones. Because this represents the largest number of devices within a network, the largest number of addresses should be allocated to these hosts. These hosts are assigned IP addresses from the range of available addresses in the network. These IP addresses can be assigned either statically or dynamically.

Static Assignment

With a static assignment, the network administrator must manually configure the network information for a host. Figure 7-29 shows the window for the network adapter properties. To configure a static IPv4 address, choose IPv4 on the network adapter screen, then key in the static address, subnet mask, and default gateway. Figure 7-30 shows the minimum static configuration: the host IP address, subnet mask, and default gateway.

Figure 7-29 Local Area Network (LAN) Interface Properties

Figure 7-30 Configuring a Static IPv4 Address

There are several advantages to static addressing. For instance, static addresses are useful for printers, servers, and other networking devices that do not change location often and need to be accessible to clients on the network based on a fixed IP address. If hosts normally access a server at a particular IP address, it would cause problems if that address changed. Additionally, static assignment of addressing information can provide increased control of network resources. For example, it is possible to create access filters based on traffic to and from a specific IP address. However, static addressing can be time consuming to enter on each host.

When using static IP addressing, it is necessary to maintain an accurate list of the IP address assigned to each device. These are permanent addresses and are not normally reused.

Assigning a Dynamic IPv4 Address to a Host (7.1.3.2)

This section discusses the use of dynamic IP addressing on a host.

Dynamic Assignment

On local networks it is often the case that the user population changes frequently. New users arrive with laptops and need a connection. Others have new workstations or other network devices, such as smartphones, that need to be connected. Rather than have the network administrator assign IP addresses for each workstation, it is easier to have IP addresses assigned automatically. This is done using the Dynamic Host Configuration Protocol (DHCP), which was introduced in Chapter 2, “Configuring a Network Operating System.” Figure 7-31 shows the configuration for obtaining a dynamic IPv4 address.

Figure 7-31 Assigning a Dynamic IPv4 Address

DHCP enables the automatic assignment of addressing information such as IP address, subnet mask, default gateway, and other configuration information. The configuration of the DHCP server requires that a block of addresses, called an address pool, be used for assigning to the DHCP clients on a network. Addresses assigned to this pool should be planned so that they exclude any static addresses used by other devices.

Using DHCP is generally the preferred method of assigning IPv4 addresses to hosts on large networks because it reduces the burden on network support staff and virtually eliminates entry errors.

Another benefit of DHCP is that an address is not permanently assigned to a host but is only “leased” for a period of time. If the host is powered down or taken off the network, the address is returned to the pool for reuse. This feature is especially helpful for mobile users that come and go on a network.

If DCHP is enabled on a host device, the ipconfig command can be used to view the IP address information assigned by the DHCP server, as shown in Figure 7-32.

Figure 7-32 Verifying a Dynamic IPv4 Address

Unicast Transmission (7.1.3.3)

In an IPv4 network, the hosts can communicate one of three ways:

Unicast: The process of sending a packet from one host to an individual host

Broadcast: The process of sending a packet from one host to all hosts in the network

Multicast: The process of sending a packet from one host to a selected group of hosts, possibly in different networks

These three types of communication are used for different purposes in data networks. In all three cases, the IPv4 address of the originating host is placed in the packet header as the source address.

Unicast Traffic

Unicast communication is used for normal host-to-host communication in both client/server and peer-to-peer networks. Unicast packets use the addresses of the destination device as the destination address and can be routed through an internetwork.

Video 7.1.3.3

View this video in the online course for an example of a unicast transmission.

In an IPv4 network, the unicast address applied to an end device is referred to as the host address. For unicast communication, the addresses assigned to the two end devices are used as the source and destination IPv4 addresses. During the encapsulation process, the source host places its IPv4 address in the unicast packet header as the source address and places the IPv4 address of the destination host in the packet header as the destination address. Regardless of whether the destination specified in a packet is unicast, broadcast, or multicast, the source address of any packet is always the unicast address of the originating host.

Note

In this course, all communications between devices is unicast communication unless otherwise noted.

IPv4 host addresses are unicast addresses and are in the address range of 0.0.0.0 to 223.255.255.255. However, within this range are many addresses that are reserved for special purposes. These special-purpose addresses will be discussed later in this chapter.

Note

The address range from 0.0.0.0 through 0.255.255.255 should not be considered part of the normal class A range. 0.x.x.x addresses serve no particular function in IP.

Broadcast Transmission (7.1.3.4)

Now that you have an understanding of how a unicast transmission works, this section examines how to reach all hosts through a broadcast transmission.

Broadcast Transmission

Broadcast traffic is used to send packets to all hosts in the network using the broadcast address for the network. With a broadcast, the packet contains a destination IP address with all 1s in the host portion. This means that all hosts on that local network (broadcast domain) will receive and look at the packet. Many network protocols, such as DHCP, use broadcasts. When a host receives a packet sent to the network broadcast address, the host processes the packet as it would a packet addressed to its unicast address.

Some examples for using broadcast transmission are

Mapping upper-layer addresses to lower-layer addresses, such as ARP

Requesting an address, such as DHCP

Unlike unicast, where the packets can be routed throughout the internetwork, broadcast packets are usually restricted to the local network. This restriction is dependent on the configuration of the gateway router and the type of broadcast. There are two types of broadcasts: directed broadcast and limited broadcast.

Directed Broadcast

A directed broadcast is sent to all hosts on a specific network. This type of broadcast is useful for sending a broadcast to all hosts on a non-local network. For example, for a host outside of the 172.16.4.0/24 network to communicate with all of the hosts within that network, the destination address of the packet would be 172.16.4.255. Although routers do not forward directed broadcasts by default, they may be configured to do so.

Limited Broadcast

The limited broadcast is used for communication that is limited to the hosts on the local network. These packets always use a destination IPv4 address of 255.255.255.255. Routers do not forward a limited broadcast. For this reason, an IPv4 network is also referred to as a broadcast domain. Routers form the boundary for a broadcast domain.

As an example, a host within the 172.16.4.0/24 network would broadcast to all hosts in its network using a packet with a destination address of 255.255.255.255.

Video 7.1.3.4

View this video in the online course for an example of a limited broadcast transmission.

When a packet is broadcast, it uses resources on the network and causes every receiving host on the network to process the packet. Therefore, broadcast traffic should be limited so that it does not adversely affect performance of the network or devices. Because routers separate broadcast domains, subdividing networks with excessive broadcast traffic can improve network performance.

Multicast Transmission (7.1.3.5)

The third type of packet transmission will be discussed in this section. IP multicast is a technique for one-to-many communication over an IP infrastructure in a network.

Multicast Transmission

Multicast transmission is designed to conserve the bandwidth of an IPv4 network. It reduces traffic by allowing a host to send a single packet to a selected set of hosts that are part of a subscribing multicast group. To reach multiple destination hosts using unicast communication, a source host would need to send an individual packet addressed to each host. With multicast, the source host can send a single packet that can reach thousands of destination hosts. The internetwork’s responsibility is to replicate the multicast flows in an efficient manner so that they reach only their intended recipients.

Some examples of multicast transmission are

Video and audio broadcasts

Routing information exchange by routing protocols

Distribution of software

News feeds

Multicast Addresses

IPv4 has a block of addresses reserved for addressing multicast groups. This address range is 224.0.0.0 to 239.255.255.255. The multicast address range is subdivided into different types of addresses: reserved link-local addresses and globally scoped addresses. One additional type of multicast address is the administratively scoped address, also called limited-scope address.

The IPv4 multicast addresses 224.0.0.0 to 224.0.0.255 are reserved link-local addresses. These addresses are to be used for multicast groups on a local network. A router connected to the local network recognizes that these packets are addressed to a link-local multicast group and never forwards them further. A typical use of reserved link-local addresses is in routing protocols using multicast transmission to exchange routing information.

The globally scoped addresses are 224.0.1.0 to 238.255.255.255. They may be used to multicast data across the Internet. For example, 224.0.1.1 has been reserved for the Network Time Protocol (NTP) to synchronize the time-of-day clocks of network devices.

The administratively scoped addresses (or limited-scope addresses) are the 239.0.0.0 to 239.255.255.255 range.

Multicast Clients

Hosts that receive particular multicast data are called multicast clients. The multicast clients use services requested by a client program to subscribe to the multicast group.

Each multicast group is represented by a single IPv4 multicast destination address. When an IPv4 host subscribes to a multicast group, the host processes packets addressed to this multicast address and packets addressed to its uniquely allocated unicast address.

Video 7.1.3.5

View this video in the online course for a demonstration of how clients accept multicast packets.

Activity 7.1.3.6: Unicast, Broadcast, or Multicast?

Go to the online course and click Start in this practice activity to view the destination IP address and see which host or hosts receive a packet based on the address.

Activity 7.1.3.7: Calculate the Network, Broadcast, and Host Addresses

Go to the online course and use the practice activity to identify the network, broadcast, and first and last usable host IP addresses.

Packet Tracer Activity 7.1.3.8: Investigate Unicast, Broadcast, and Multicast Traffic

In this activity you will examine unicast, broadcast, and multicast behavior. Most traffic in a network is unicast. When a PC sends an ICMP echo request to a remote router, the source address in the IP packet header is the IP address of the sending PC. The destination address in the IP packet header is the IP address of the interface on the remote router. The packet is sent only to the intended destination.

Using the ping command or the Add Complex PDU feature of Packet Tracer, you can directly ping broadcast addresses to view broadcast traffic.

For multicast traffic, you will view EIGRP traffic.

Types of IPv4 Addresses (7.1.4)

IPv4 designates several different types of IPv4 addresses, as described in this section.

Public and Private IPv4 Addresses (7.1.4.1)

Although most IPv4 host addresses are public addresses designated for use in networks that are accessible on the Internet, there are blocks of addresses that are used in networks that require limited or no Internet access. These addresses are called private addresses.

Private Addresses

The private address blocks are

10.0.0.0 to 10.255.255.255 (10.0.0.0/8)

172.16.0.0 to 172.31.255.255 (172.16.0.0/12)

192.168.0.0 to 192.168.255.255 (192.168.0.0/16)

Private addresses are defined in RFC 1918, Address Allocation for Private Internets, and are sometimes referred to as RFC 1918 addresses. Private space address blocks, as shown in Figure 7-33, are used in private networks. Hosts that do not require access to the Internet can use private addresses. However, within the private network, hosts still require unique IP addresses within the private space.

Figure 7-33 Private Addresses Cannot Be Routed over the Internet

Hosts in different networks may use the same private space addresses. Packets using these addresses as the source or destination should not appear on the public Internet. The router or firewall device at the perimeter of these private networks must block or translate these addresses. Even if these packets were to make their way to the Internet, the routers would not have routes to forward them to the appropriate private network.

In RFC 6598, the Internet Assigned Numbers Authority (IANA) reserved another group of addresses known as shared address space. Similar to RFC 1918 private address space, shared address space addresses are not globally routable. However, these addresses are intended only for use in service provider networks. The shared address block is 100.64.0.0/10.

Public Addresses

The vast majority of the addresses in the IPv4 unicast host range are public addresses, which are designed to be used in the hosts that are publicly accessible from the Internet. Even within these IPv4 address blocks, there are many addresses that are designated for other special purposes.

Activity 7.1.4.2: Pass or Block IPv4 Addresses

Go to the online course and click on start. You will then have to either click the Pass or Block buttons based on whether it is a Public (Pass) or Private address (Block).

Special-Use IPv4 Addresses (7.1.4.3)

There are certain addresses that cannot be assigned to hosts. There are also special-use addresses that can be assigned to hosts, but with restrictions on how those hosts can interact within the network.

Network and Broadcast Addresses

As explained earlier, within each network the first and last addresses cannot be assigned to hosts. These are the network address and the broadcast address, respectively.

Loopback

One such reserved address is the IPv4 loopback address, 127.0.0.1. Often simply called the loopback, this is a special address that hosts use to direct traffic to themselves. The loopback address creates a shortcut method for TCP/IP applications and services that run on the same device to communicate with one another. By using the loopback address instead of the assigned IPv4 host address, two services on the same host can bypass the lower layers of the TCP/IP stack. You can also ping the loopback address to test the configuration of TCP/IP on the local host.

Although only the single 127.0.0.1 address is used, addresses 127.0.0.0 to 127.255.255.255 are reserved. Any address within this block will loop back to the local host. No address within this block should ever appear on any network.

Link-Local Addresses

IPv4 addresses in the address block 169.254.0.0 to 169.254.255.255 (169.254.0.0/16) are designated as link-local addresses. These addresses can be automatically assigned to the local host by the operating system in environments where no IP configuration is available. These might be used in a small peer-to-peer network or for a host that could not automatically obtain an address from a DHCP server.

Communication using IPv4 link-local addresses is only suitable for communication with other devices connected to the same network, as shown in Figure 7-34. A host must not send a packet with an IPv4 link-local destination address to any router for forwarding and should set the IPv4 Time-to-Live (TTL) field for these packets to 1.

Figure 7-34 Special IPv4 Addresses

Link-local addresses do not provide services outside of the local network. However, many client/server and peer-to-peer applications will work properly with IPv4 link-local addresses.

TEST-NET Addresses

The address block 192.0.2.0 to 192.0.2.255 (192.0.2.0/24) is set aside for teaching and learning purposes. These TEST-NET addresses can be used in documentation and network examples. Unlike the experimental addresses, network devices will accept these addresses in their configurations. You may often find these addresses used with the domain names example.com or example.net in RFCs, vendor documentation, and protocol documentation. Addresses within this block should not appear on the Internet.

Experimental Addresses

The addresses in the block 240.0.0.0 to 255.255.255.254 are listed as reserved for future use (RFC 3330). Currently, these addresses can be used only for research or experimentation purposes, and cannot be used in an IPv4 network. Though, according to RFC 3330, they could, technically, be converted to usable addresses in the future.

Legacy Classful Addressing (7.1.4.4)

Historically, RFC 1700, Assigned Numbers, grouped the unicast ranges into specific sizes called class A, class B, and class C addresses. It also defined class D (multicast) and class E (experimental) addresses, as previously presented. The unicast address classes A, B, and C defined specifically sized networks and specific address blocks for these networks. A company or organization was assigned an entire network from a class A, class B, or class C address block. This use of address space is referred to as classful addressing.

Class A Blocks

A class A address block was designed to support extremely large networks with more than 16 million host addresses. Class A IPv4 addresses used a fixed /8 prefix with the first octet to indicate the network address. The remaining three octets were used for host addresses. All class A addresses required that the most significant bit of the high-order octet (the leftmost bit of the 32 bits) be a 0. This meant that there were only 128 possible class A networks, 0.0.0.0/8 to 127.0.0.0/8. Even though the class A addresses reserved one-half of the address space, because of their limit of 128 networks, they could only be allocated to approximately 120 companies or organizations.

Class B Blocks

Class B address space was designed to support the needs of midsize to large networks with up to approximately 65,000 hosts. A class B IP address used the two high-order octets to indicate the network address. The other two octets specified host addresses. For class B addresses, the most significant two bits of the high-order octet were 10. This restricted the address block for class B to 128.0.0.0/16 to 191.255.0.0/16. Class B had slightly more efficient allocation of addresses than class A because it equally divided 25% of the total IPv4 address space among approximately 16,000 networks.

Class C Blocks

The class C address space was the most commonly available of the historic address classes. This address space was intended to provide addresses for small networks with a maximum of 254 hosts. Class C address blocks used a /24 prefix. This meant that a class C network used only the last octet as host addresses, with the three high-order octets used to indicate the network address. Class C address blocks set aside address space by using a fixed value of 110 for the three most significant bits of the high-order octet. This restricted the address block for class C from 192.0.0.0/24 to 223.255.255.0/24. Although it occupied only 12.5% of the total IPv4 address space, it could provide addresses to 2 million networks.

Figure 7-35 illustrates how these address classes are divided.

Figure 7-35 Address Classes

Limits to the Class-Based System

Not all organizations’ requirements fit well into one of these three classes. Classful allocation of address space often wasted many addresses, which exhausted the availability of IPv4 addresses. For example, a company that had a network with 260 hosts would need to be given a class B address with more than 65,000 addresses.

Even though this classful system was all but abandoned in the late 1990s, you will see remnants of it in networks today. For example, when you assign an IPv4 address to a computer, the operating system examines the address being assigned to determine if this address is a class A, class B, or class C address. The operating system then assumes the prefix used by that class and makes the default subnet mask assignment.

Figure 7-36 shows the classful address ranges.

Figure 7-36 Classful Address Ranges

Classless Addressing

The system in use today is referred to as classless addressing. The formal name is Classless Inter-Domain Routing (CIDR, pronounced “cider”). The classful allocation of IPv4 addresses was very inefficient, allowing for only /8, /16, or /24 prefix lengths, each from a separate address space. In 1993, the Internet Engineering Task Force (IETF) created a new set of standards that allowed service providers to allocate IPv4 addresses on any address bit boundary (prefix length) instead of only by a class A, B, or C address.

The IETF knew that CIDR was only a temporary solution and that a new IP protocol would have to be developed to accommodate the rapid growth in the number of Internet users. In 1994, the IETF began its work to find a successor to IPv4, which eventually became IPv6.

Assignment of IP Addresses (7.1.4.5, 7.1.4.6)

For a company or organization to have network hosts, such as web servers, accessible from the Internet, that organization must have a block of public addresses assigned. Remember that public addresses must be unique, and use of these public addresses is regulated and allocated to each organization separately. This is true for IPv4 and IPv6 addresses.

IANA and RIRs

The Internet Assigned Numbers Authority (IANA) (http://www.iana.org) manages the allocation of IPv4 and IPv6 addresses. Until the mid-1990s, all IPv4 address space was managed directly by IANA. At that time, the remaining IPv4 address space was allocated to various other registries to manage for particular purposes or for regional areas. These registration companies are called Regional Internet Registries (RIRs) and their general regions are depicted in Figure 7-37.

Figure 7-37 Assignment of IP Addresses by Regional Internet Registry (RIR)

The following are the major RIRs, the regions they cover, and their URLs:

AfriNIC (African Network Information Centre): Africa Region; http://www.afrinic.net

APNIC (Asia Pacific Network Information Centre): Asia/Pacific Region; http://www.apnic.net

ARIN (American Registry for Internet Numbers): North America Region; http://www.arin.net

LACNIC (Regional Latin-American and Caribbean IP Address Registry): Latin America and some Caribbean Islands; http://www.lacnic.net

RIPE NCC (Reseaux IP Europeans): Europe, the Middle East, and Central Asia; http://www.ripe.net

ISPs

RIRs are responsible for allocating IP addresses to the Internet service providers (ISPs). Most companies or organizations obtain their IPv4 address blocks from an ISP. An ISP will generally supply a small number of usable IPv4 addresses (8 or 16) to their customers as a part of their services. Larger blocks of addresses can be obtained based on justification of needs and for additional service costs.

In a sense, the ISP loans or rents these addresses to the organization. If we choose to move our Internet connectivity to another ISP, the new ISP will provide us with addresses from the address blocks that have been provided to them, and our previous ISP returns the blocks loaned to us to their allocation to be loaned to another customer.

IPv6 addresses can be obtained from the ISP or, in some cases, directly from the RIR. IPv6 addresses and typical address block sizes will be discussed later in this chapter.

ISP Services

To get access to the services of the Internet, we have to connect our data network to the Internet using an ISP. ISPs have their own set of internal data networks to manage Internet connectivity and to provide related services. Among the other services that an ISP generally provides to its customers are DNS services, e-mail services, and a website. Depending on the level of service required and available, customers use different tiers of an ISP.

ISP Tiers

ISPs are designated by a hierarchy based on their level of connectivity to the Internet backbone. Each lower tier obtains connectivity to the backbone via a connection to a higher-tier ISP, as shown in Figure 7-38.

Figure 7-38 Three Tiers of ISP

Tier 1

As shown in Figure 7-38, at the top of the ISP hierarchy are Tier 1 ISPs. These ISPs are large national or international ISPs that are directly connected to the Internet backbone. The customers of Tier 1 ISPs are either lower-tiered ISPs or large companies and organizations. Because they are at the top of Internet connectivity, Tier 1 ISPs engineer highly reliable connections and services. Among the technologies used to support this reliability are multiple connections to the Internet backbone.

The primary advantages for customers of Tier 1 ISPs are reliability and speed. Because these customers are only one connection away from the Internet, there are fewer opportunities for failures or traffic bottlenecks. The drawback for Tier 1 ISP customers is its high cost.

Tier 2

As shown in Figure 7-38, Tier 2 ISPs acquire their Internet service from Tier 1 ISPs. Tier 2 ISPs generally focus on business customers. Tier 2 ISPs usually offer more services than the other two tiers of ISPs. These Tier 2 ISPs tend to have the information technology (IT) resources to operate their own services such as Domain Name Service (DNS), e-mail servers, and web servers. Other services that Tier 2 ISPs may offer include website development and maintenance, e-commerce/e-business, and Voice over Internet Protocol (VoIP).

The primary disadvantage of Tier 2 ISPs, as compared to Tier 1 ISPs, is slower Internet access. Because Tier 2 ISPs are at least one more connection away from the Internet backbone, they also tend to have lower reliability than Tier 1 ISPs.

Tier 3

As shown in Figure 7-38, Tier 3 ISPs purchase their Internet service from Tier 2 ISPs. The focus of these ISPs is the retail and home markets in a specific locale. Tier 3 customers typically do not need many of the services required by Tier 2 customers. Their primary need is connectivity and support.

These customers often have little or no computer or network expertise. Tier 3 ISPs often bundle Internet connectivity as a part of network and computer service contracts for their customers. Although they may have reduced bandwidth and less reliability than Tier 1 and Tier 2 providers, Tier 3 ISPs are often good choices for small to midsize companies.

Activity 7.1.4.7: Public or Private IPv4 Address

Go to the online course to drag each IP address to the correct category, Public or Private.

Lab 7.1.4.8: Identifying IPv4 Addresses

In this lab you will identify and classify IPv4 addresses.

IPv6 Network Addresses (7.2)

Internet Protocol version 6 (IPv6) is the latest revision of the Internet Protocol. This section introduces the new protocol and explains why it is needed.

IPv4 Issues (7.2.1)

This section discusses some of the reasons why IPv4 needs to be updated to IPv6.

The Need for IPv6 (7.2.1.1)

IPv6 is designed to be the successor to IPv4. IPv6 has a larger, 128-bit address space, providing for 340 undecillion addresses. (That is the number 340, followed by 36 zeros.) However, IPv6 is much more than just larger addresses. When the IETF began its development of a successor to IPv4, it used this opportunity to fix the limitations of IPv4 and include additional enhancements. One example is Internet Control Message Protocol version 6 (ICMPv6), which includes address resolution and address auto-configuration not found in ICMP for IPv4 (ICMPv4). ICMPv4 and ICMPv6 are discussed later in this chapter.

IPv4 Address Depletion

The depletion of IPv4 address space has been the motivating factor for moving to IPv6. As Africa, Asia, and other areas of the world become more connected to the Internet, there are not enough IPv4 addresses to accommodate this growth. On Monday, January 31, 2011, IANA allocated the last two /8 IPv4 address blocks to the RIRs. Various projections show that all five RIRs will have run out of IPv4 addresses between 2015 and 2020. At that point, the remaining IPv4 addresses will have been allocated to ISPs.

IPv4 has a theoretical maximum of 4.3 billion addresses. RFC 1918 private addresses in combination with network address translation (NAT) have been instrumental in slowing the depletion of IPv4 address space. NAT has limitations that severely impede peer-to-peer communications.

Internet of Things

Figure 7-39 shows the changes through the years in the number of things connected by the Internet.

Figure 7-39 Internet of Things

The Internet of today is significantly different than the Internet of past decades. The Internet of today is more than e-mail, web pages, and file transfer between computers. The evolving Internet is becoming an Internet of things. As depicted in Figure 7-39, no longer will the only devices accessing the Internet be computers, tablets, and smartphones. The sensor-equipped, Internet-ready devices of tomorrow will include everything from automobiles and biomedical devices to household appliances and natural ecosystems. Imagine a meeting at a customer site that is automatically scheduled on your calendar application to begin an hour before you normally start work. This could be a significant problem, especially if you forget to check the calendar or adjust the alarm clock accordingly. Now imagine that the calendar application communicates this information directly to your alarm clock and to your automobile. Your alarm clock wakes you up in plenty of time to get ready for your meeting, and your car automatically warms up to melt the ice on the windshield before you enter the car, and then gives you instructions for which route to take to your meeting.

With an increasing Internet population, a limited IPv4 address space, issues with NAT, and an Internet of things, the time has come to begin the transition to IPv6.

IPv4 and IPv6 Coexistence (7.2.1.2)

There is not a single date to move to IPv6. For the foreseeable future, both IPv4 and IPv6 will coexist. The transition is expected to take years. The IETF has created various protocols and tools to help network administrators migrate their networks to IPv6. The migration techniques can be divided into three categories:

Dual stack: As shown in Figure 7-40, a dual stack allows IPv4 and IPv6 to coexist on the same network. Dual-stack devices run both IPv4 and IPv6 protocol stacks simultaneously.

Figure 7-40 Dual Stack

Tunneling: As shown in Figure 7-41, tunneling is a method of transporting an IPv6 packet over an IPv4 network. The IPv6 packet is encapsulated inside an IPv4 packet, similar to other types of data.

Figure 7-41 Tunneling

Translation: As shown in Figure 7-42, Network Address Translation 64 (NAT64) allows IPv6-enabled devices to communicate with IPv4-enabled devices using a translation technique similar to NAT for IPv4. An IPv6 packet is translated to an IPv4 packet, and vice versa.

Figure 7-42 Translation

Activity 7.2.1.3: IPv4 and IPv6 Terms and Descriptions

Go to the online course to complete the activity. This activity is designed to have you drag and drop the provided IPv4 and IPv6 term to the field next to the appropriate description

IPv6 Addressing (7.2.2)

This section looks at the process of IPv6 addressing.

Hexadecimal Number System (7.2.2.1)

Unlike IPv4 addresses that are expressed in dotted decimal notation, IPv6 addresses are represented using hexadecimal values. You have seen hexadecimal used in the Packets Byte pane of Wireshark. In Wireshark, hexadecimal is used to represent the binary values within frames and packets. Hexadecimal is also used to represent Ethernet Media Access Control (MAC) addresses.

Hexadecimal Numbering

Hexadecimal (“Hex”) is a convenient way to represent binary values. Just as decimal is a base 10 numbering system and binary is base 2, hexadecimal is a base 16 numbering system.

The base 16 numbering system uses the numbers 0 to 9 and the letters A to F. Figure 7-43 shows the equivalent decimal, binary, and hexadecimal values. There are 16 unique combinations of 4 bits, from 0000 to 1111. The 16-digit hexadecimal numbering system is the perfect numbering system to use, because any 4 bits can be represented with a single hexadecimal value.

Figure 7-43 Hexadecimal Numbering

Understanding Bytes

Given that 8 bits (1 byte) is a common binary grouping, binary 00000000 to 11111111 can be represented in hexadecimal as the range 00 to FF. Leading zeros can be displayed to complete the 8-bit representation. For example, the binary value 0000 1010 is shown in hexadecimal as 0A.

Representing Hexadecimal Values

Hexadecimal is usually represented in text by the value preceded by 0x (for example, 0x73) or a subscript 16. Less commonly, it may be followed by an H (for example, 73H). However, because subscript text is not recognized in command-line or programming environments, the technical representation of hexadecimal is preceded with “0x” (zero X). Therefore, the earlier examples would be shown as 0x0A and 0x73, respectively.

Note

It is important to distinguish hexadecimal values from decimal values regarding the characters 0 to 9.

Hexadecimal Conversions

Number conversions between decimal and hexadecimal values are straightforward, but quickly dividing or multiplying by 16 is not always convenient.

With practice, it is possible to recognize the binary bit patterns that match the decimal and hexadecimal values. Figure 7-44 shows these patterns for selected 8-bit values.

Figure 7-44 Hexadecimal Conversion Patterns

IPv6 Address Representation (7.2.2.2)

IPv6 addresses are 128 bits in length and written as a string of hexadecimal values. Every 4 bits is represented by a single hexadecimal digit, for a total of 32 hexadecimal values. IPv6 addresses are not case sensitive and can be written in either lowercase or uppercase.

Preferred Format

As shown in Figure 7-45, the preferred format for writing an IPv6 address is x:x:x:x:x:x:x:x, with each “x” consisting of four hexadecimal values. In IPv6, a hextet is the unofficial term used to refer to a segment of 16 bits or four hexadecimal values. Each “x” is a single hextet, 16 bits or four hexadecimal digits.

Figure 7-45 Hextets

“Preferred format” means the IPv6 address is written using all 32 hexadecimal digits. It does not necessarily mean it is the ideal method for representing the IPv6 address. In the following pages, we will see two rules to help reduce the number of digits needed to represent an IPv6 address. Figure 7-46 has examples of IPv6 addresses in the preferred format.

Figure 7-46 Preferred Format Examples

Rule 1: Omitting Leading 0s (7.2.2.3)

The first rule to help reduce the notation of IPv6 addresses is that any leading 0s in any 16-bit section or hextet can be omitted. For example:

01AB can be represented as 1AB.

09F0 can be represented as 9F0.

0A00 can be represented as A00.

00AB can be represented as AB.

This rule only applies to leading 0s, not to trailing 0s; otherwise the address would be ambiguous. For example, the hextet ABC could be either 0ABC or ABC0.

Figures 7-47 through 7-54 show several examples of how omitting leading 0s can be used to reduce the size of an IPv6 address. For each example the preferred format is shown. Notice how omitting the leading 0s in all examples results in a smaller address representation.

Figure 7-47 Omitting Leading 0s: Example 1

Figure 7-48 Omitting Leading 0s: Example 2

Figure 7-49 Omitting Leading 0s: Example 3

Figure 7-50 Omitting Leading 0s: Example 4

Figure 7-51 Omitting Leading 0s: Example 5

Figure 7-52 Omitting Leading 0s: Example 6

Figure 7-53 Omitting Leading 0s: Example 7

Figure 7-54 Omitting Leading 0s: Example 8

Rule 2: Omitting All 0 Segments (7.2.2.4)

The second rule to help reduce the notation of IPv6 addresses is that a double colon (::) can replace any single, contiguous string of one or more 16-bit segments (hextets) consisting of all 0s.

The double colon (::) can be used only once within an address; otherwise there would be more than one possible resulting address. When used with the omitting leading 0s technique, the notation of IPv6 address can often be greatly reduced. This is commonly known as the compressed format.

The following address is incorrect because it includes two sets of double colons:

2001:0DB8::ABCD::1234

Possible expansions for the preceding ambiguous compressed address include the following:

2001:0DB8::ABCD:0000:0000:1234

2001:0DB8::ABCD:0000:0000:0000:1234

2001:0DB8:0000:ABCD::1234

2001:0DB8:0000:0000:ABCD::1234

Figures 7-55 through 7-61 show several examples of how using the double colon (::) and omitting leading 0s can reduce the size of an IPv6 address.

Figure 7-55 Using Double Colons: Example 1

Figure 7-56 Using Double Colons: Example 2

Figure 7-57 Using Double Colons: Example 3

Figure 7-58 Using Double Colons: Example 4

Figure 7-59 Using Double Colons: Example 5

Figure 7-60 Using Double Colons: Example 6

Figure 7-61 Using Double Colons: Example 7

Activity 7.2.2.5: Practicing IPv6 Address Representations

Go to the online course to complete the activity. This activity provides practice in converting IPv6 addresses into short and compressed forms.

Types of IPv6 Addresses (7.2.3)

This section takes a closer look at the different types of IPv6 addresses.

IPv6 Address Types (7.2.3.1)

There are three types of IPv6 addresses:

Unicast: An IPv6 unicast address uniquely identifies an interface on an IPv6-enabled device. As shown in Figure 7-62, a source IPv6 address must be a unicast address.

Figure 7-62 IPv6 Unicast Communications

Multicast: An IPv6 multicast address is used to send a single IPv6 packet to multiple destinations.

Anycast: An IPv6 anycast address is any IPv6 unicast address that can be assigned to multiple devices. A packet sent to an anycast address is routed to the nearest device having that address. Anycast addresses are beyond the scope of this course.

Unlike IPv4, IPv6 does not have a broadcast address. However, there is an IPv6 all-nodes multicast address that essentially gives the same result.

IPv6 Prefix Length (7.2.3.2)

Recall that the prefix, or network, portion of an IPv4 address can be identified by a dotted-decimal subnet mask or prefix length (slash notation). For example, an IP address of 192.168.1.10 with dotted-decimal subnet mask 255.255.255.0 is equivalent to 192.168.1.10/24.

IPv6 uses the prefix length to represent the prefix portion of the address. IPv6 does not use the dotted-decimal subnet mask notation. The prefix length is used to indicate the network portion of an IPv6 address using the IPv6 address/prefix length.

The prefix length can range from 0 to 128. A typical IPv6 prefix length for LANs and most other types of networks is /64. This means the prefix or network portion of the address is 64 bits in length, leaving another 64 bits for the Interface ID (host portion) of the address, as shown in Figure 7-63.

Figure 7-63 /64 Prefix

IPv6 Unicast Addresses (7.2.3.3)

An IPv6 unicast address uniquely identifies an interface on an IPv6-enabled device. A packet sent to a unicast address is received by the interface that is assigned that address. Similar to IPv4, a source IPv6 address must be a unicast address. The destination IPv6 address can be either a unicast address or a multicast address.

There are six types of IPv6 unicast addresses, as shown in Figure 7-64 and described in the following list.

Figure 7-64 IPv6 Unicast Addresses

Global unicast: A global unicast address is similar to a public IPv4 address. These are globally unique, Internet-routable addresses. Global unicast addresses can be configured statically or assigned dynamically. There are some important differences in how a device receives its IPv6 address dynamically compared to DHCP for IPv4.

Link-local: Link-local addresses are used to communicate with other devices on the same local link. With IPv6, the term link refers to a subnet. Link-local addresses are confined to a single link. Their uniqueness must be confirmed on that link only because they are not routable beyond the link. In other words, routers will not forward packets with a link-local source or destination address.

Loopback: The loopback address is used by a host to send a packet to itself and cannot be assigned to a physical interface. Similar to an IPv4 loopback address, you can ping an IPv6 loopback address to test the configuration of TCP/IP on the local host. The IPv6 loopback address is all 0s except for the last bit, represented as ::1/128 or just ::1 in the compressed format.

Unspecified address: An unspecified address is an all-0s address represented as ::/128 or just :: in the compressed format. It cannot be assigned to an interface and can only be used as a source address in an IPv6 packet. An unspecified address is used as a source address when the device does not yet have a permanent IPv6 address or when the source of the packet is irrelevant to the destination.

Unique local: IPv6 unique local addresses have some similarity to RFC 1918 private addresses for IPv4, but there are significant differences as well. Unique local addresses are used for local addressing within a site or between a limited number of sites. These addresses should not be routable in the global IPv6. Unique local addresses are in the range of FC00::/7 to FDFF::/7.

With IPv4, private addresses are combinedindenthangingN with NAT/PAT to provide a many-to-one translation of private-to-public addresses. This is done because of the limited availability of IPv4 address space. Many sites also use the private nature of RFC 1918 addresses to help secure or hide their network from potential security risks. However, this was never the intended use of these technologies, and the IETF has always recommended that sites take the proper security precautions on their Internet-facing routers. Although IPv6 does provide for site-specific addressing, it is not intended to be used to help hide internal IPv6-enabled devices from the IPv6 Internet. IETF recommends that limiting access to devices should be accomplished using proper, best-practice security measures.

Note

The original IPv6 specification defined site-local addresses for a similar purpose, using the prefix range FEC0::/10. There were several ambiguities in the specification, and site-local addresses were deprecated by the IETF in favor of unique local addresses.

Embedded IPv4: The last type of unicast address is the IPv4 embedded address. These addresses are used to help transition from IPv4 to IPv6. IPv4 embedded addresses are beyond the scope of this course.

IPv6 Link-Local Unicast Addresses (7.2.3.4)

An IPv6 link-local address enables a device to communicate with other IPv6-enabled devices on the same link, and only on that link (subnet). Packets with a source or destination link-local address cannot be routed beyond the link from where the packet originated.

Unlike IPv4 link-local addresses, IPv6 link-local addresses have a significant role in various aspects of the network. The global unicast address is not a requirement; however, every IPv6-enabled network interface is required to have a link-local address.

If a link-local address is not configured manually on an interface, the device will automatically create its own without communicating with a DHCP server. IPv6-enabled hosts create an IPv6 link-local address even if the device has not been assigned a global unicast IPv6 address. This allows IPv6-enabled devices to communicate with other IPv6-enabled devices on the same subnet. This includes communication with the default gateway (router).

IPv6 link-local addresses are in the FE80::/10 range. The /10 indicates that the first 10 bits are 1111 1110 10xx xxxx. The first hextet has a range of 1111 1110 1000 0000 (FE80) to 1111 1110 1011 1111 (FEBF).

Figure 7-65 shows an example of communication using IPv6 link-local addresses. Figure 7-66 shows the format of an IPv6 link-local address.

Figure 7-65 IPv6 Link-Local Communications

Figure 7-66 IPv6 Link-Local Address

IPv6 link-local addresses are also used by IPv6 routing protocols to exchange messages and as the next-hop address in the IPv6 routing table

Note

Typically, it is the link-local address of the router, not the global unicast address, that is used as the default gateway for other devices on the link.

Activity 7.2.3.5: Identify Types of IPv6 Addresses

Go to the online course to complete the drag-and-drop activity by dragging the IPv6 address type to the most appropriate description.

IPv6 Unicast Addresses (7.2.4)

This section looks at the structure, static and dynamic configuration of IPv6 unicast addresses. The section also discusses the static and dynamic link-local address, as well as how to verify IPv6 configuration.

Structure of an IPv6 Global Unicast Address (7.2.4.1)

IPv6 global unicast addresses are globally unique and routable on the IPv6 Internet. These addresses are equivalent to public IPv4 addresses. The Internet Committee for Assigned Names and Numbers (ICANN), the operator for IANA, allocates IPv6 address blocks to the five RIRs. Currently, only global unicast addresses with the first three bits of 001 or 2000::/3 are being assigned. This is only 1/8th of the total available IPv6 address space, excluding only a very small portion for other types of unicast and multicast addresses. Figure 7-67 shows the structure and range of a global unicast address.

Figure 7-67 IPv6 Global Unicast Address

Note

The 2001:0DB8::/32 address has been reserved for documentation purposes, including use in examples.

A global unicast address has three parts:

Global routing prefix: The global routing prefix is the prefix, or network, portion of the address that is assigned by the provider, such as an ISP, to a customer or site. Currently, RIRs assign a /48 global routing prefix to customers. This includes everyone from enterprise business networks to individual households. This is more than enough address space for most customers.

Figure 7-68 shows the structure of a global unicast address using a /48 global routing prefix. /48 prefixes are the most common global routing prefixes assigned and will be used in most of the examples throughout this course.

Figure 7-68 IPv6 /48 Global Routing Prefix

For example, the IPv6 address 2001:0DB8:ACAD::/48 has a prefix that indicates that the first 48 bits (three hextets) (2001:0DB8:ACAD) is the prefix, or network, portion of the address. The double colon (::) prior to the /48 prefix length means the rest of the address contains all 0s.

Subnet ID: The Subnet ID is used by an organization to identify subnets within its site.

Interface ID: The IPv6 Interface ID is equivalent to the host portion of an IPv4 address. The term Interface ID is used because a single host may have multiple interfaces, each having one or more IPv6 addresses.

Note

Unlike IPv4, in IPv6 the all-0s address can be assigned to a device because there are no broadcast addresses in IPv6. However, the all-0s address is reserved as a Subnet-Router anycast address, and should be assigned only to routers.

An easy way to read most IPv6 addresses is to count the number of hextets. As shown in Figure 7-69, in a /64 global unicast address the first four hextets are for the network portion of the address, with the fourth hextet indicating the Subnet ID. The remaining four hextets are for the Interface ID.

Figure 7-69 IPv6 Reading a Global Unicast Address

Static Configuration of a Global Unicast Address (7.2.4.2)

This section provides procedures for setting up global unicast addresses on devices.

Router Configuration

Most IPv6 configuration and verification commands in the Cisco IOS software are similar to their IPv4 counterparts. In many cases the only difference is the use of ipv6 in place of ip within the commands.

The interface command to configure an IPv6 global unicast address on an interface is ipv6 address ipv6-address/prefix-length.

Notice that there is not a space between ipv6-address and prefix-length.

The example configuration will use the topology shown in Figure 7-70 and these IPv6 subnets:

2001:0DB8:ACAD:0001:/64 (or 2001:DB8:ACAD:1::/64)

2001:0DB8:ACAD:0002:/64 (or 2001:DB8:ACAD:2::/64)

2001:0DB8:ACAD:0003:/64 (or 2001:DB8:ACAD:3::/64)

Figure 7-70 Configuring IPv6 on a Router

As shown in Figure 7-71, configuring the IPv6 global unicast address on the Gigabit Ethernet 0/0 (G0/0) interface of R1 would require the following commands:

Click here to view code image

Router(config)# interface GigabitEthernet 0/0
Router(config-if)# ipv6 address 2001:db8:acad:1::1/64
Router(config-if)# no shutdown

Figure 7-71 Configuring IPv6 on a Router Interface

Host Configuration

Manually configuring the IPv6 address on a host is similar to configuring an IPv4 address.

As shown in Figure 7-72, the default gateway address configured for PC1 is 2001:DB8:ACAD:1::1, the global unicast address of the R1 Gigabit Ethernet interface on the same network.

Figure 7-72 Configuring IPv6 on a Host

Activity 7.2.4.2: Syntax Checker

Go to the online course to use the Syntax Checker to configure the IPv6 global unicast address.

Just as with IPv4, configuring static addresses on clients does not scale to larger environments. For this reason, most network administrators in an IPv6 network will enable dynamic assignment of IPv6 addresses.

There are two ways in which a device can obtain an IPv6 global unicast address automatically:

Stateless Address Autoconfiguration (SLAAC)

DHCPv6

Dynamic Configuration of a Global Unicast Address Using SLAAC (7.2.4.3)

This section examines how Stateless Address Autoconfiguration (SLAAC) is used in the configuration of a global unicast address.

Stateless Address Autoconfiguration

Stateless Address Autoconfiguration (SLAAC) is a method that allows a device to obtain its prefix, prefix length, and default gateway address information from an IPv6 router without the use of a DHCPv6 server. Using SLAAC, devices rely on the local router’s ICMPv6 Router Advertisement (RA) messages to obtain the necessary information.

IPv6 routers periodically send out ICMPv6 RA messages to all IPv6-enabled devices on the network. By default, Cisco routers send out RA messages every 200 seconds to the IPv6 all-nodes multicast group address. An IPv6 device on the network does not have to wait for these periodic RA messages. A device can send a Router Solicitation (RS) message to the router, using the IPv6 all-routers multicast group address. When an IPv6 router receives an RS message, it will immediately respond with an RA.

Even though an interface on a Cisco router can be configured with an IPv6 address, this does not make it an “IPv6 router.” An IPv6 router is a router that

Forwards IPv6 packets between networks

Can be configured with static IPv6 routes or a dynamic IPv6 routing protocol

Sends ICMPv6 RA messages

IPv6 routing is not enabled by default. To enable a router as an IPv6 router, the ipv6 unicast-routing global configuration command must be used.

Note

IPv4 is enabled on Cisco routers by default.

The ICMPv6 RA message contains the prefix, prefix length, and other information for the IPv6 device. The RA message also informs the IPv6 device how to obtain its addressing information. The RA message can contain one of the following three options, as shown in Figure 7-73:

Figure 7-73 Router Solicitation and Router Advertisement Messages

Option 1 – SLAAC Only: The device should use the prefix, prefix-length, and default gateway address information contained in the RA message. No other information is available from a DHCPv6 server.

Option 2 – SLAAC and DHCPv6: The device should use the prefix, prefix-length, and default gateway address information in the RA message. Other information is available from a DHCPv6 server, such as the DNS server address. The device will, through the normal process of discovering and querying a DHCPv6 server, obtain this additional information. This is known as stateless DHCPv6 because the DHCPv6 server does not need to allocate or keep track of any IPv6 address assignments; it only needs to provide additional information, such as the DNS server address.

Option 3 – DHCPv6 Only: The device should not use the information in this RA message for its addressing information. Instead, the device should use the normal process of discovering and querying a DHCPv6 server to obtain all of its addressing information. This includes an IPv6 global unicast address, prefix length, a default gateway address, and the addresses of DNS servers. In this case, the DHCPv6 server is acting as a stateful DHCP server similar to DHCP for IPv4. The DHCPv6 server allocates and keeps track of IPv6 addresses so it does not assign the same IPv6 address to multiple devices.

Routers send ICMPv6 RA messages using the link-local address as the source IPv6 address. Devices using SLAAC use the router’s link-local address as their default gateway address.

Dynamic Configuration of a Global Unicast Address Using DHCPv6 (7.2.4.4)

The other option for dynamically configuring a global unicast address is to use DHCPv6, as described in this section.

DHCPv6

Dynamic Host Configuration Protocol for IPv6 (DHCPv6) is similar to DHCP for IPv4. A device can automatically receive its addressing information, including a global unicast address, prefix length, default gateway address, and the addresses of DNS servers, by using the services of a DHCPv6 server.

A device may receive all or some of its IPv6 addressing information from a DHCPv6 server, depending upon whether option 2 (SLAAC and DHCPv6) or option 3 (DHCPv6 Only) is specified in the ICMPv6 RA message. Additionally, the host OS may choose to ignore whatever is in the router’s RA message and obtain its IPv6 address and other information directly from a DHCPv6 server, as shown in Figure 7-74.

Figure 7-74 Router Solicitation and Router Advertisement Messages (DHCPv6)

Before deploying IPv6 devices in a network, it is a good idea to first verify whether the host observes the options within the router’s ICMPv6 RA message.

A device may obtain its IPv6 global unicast address dynamically and also be configured with multiple static IPv6 addresses on the same interface. IPv6 allows for multiple IPv6 addresses, belonging to the same IPv6 network, to be configured on the same interface.

A device may also be configured with more than one default gateway IPv6 address. For further information about how the decision is made regarding which address is used as a source IPv6 address or which default gateway address is used, refer to RFC 6724, Default Address Selection for IPv6.

The Interface ID

If the client does not use the information contained within the RA message and relies solely on DHCPv6, then the DHCPv6 server will provide the entire IPv6 global unicast address, including the prefix and the Interface ID.

EUI-64 Process or Randomly Generated (7.2.4.5)

If we use SLAAC Only or SLAAC with DHCPv6, the client does not obtain the actual Interface ID portion of the address. The client device must determine its own 64-bit Interface ID, either by using the EUI-64 process or by generating a random 64-bit number.

This section looks at the Extended Unique Identifier (EUI-64) process, and the randomly generated identifier that does not use the MAC address.

EUI-64 Process

IEEE defined the Extended Unique Identifier (EUI) or modified EUI-64 process. This process uses a client’s 48-bit Ethernet MAC address and inserts another 16 bits in the middle of the 48-bit MAC address to create a 64-bit Interface ID.

Ethernet MAC addresses are usually represented in hexadecimal and are made up of two parts:

Organizationally Unique Identifier (OUI): The OUI is a 24-bit (six hexadecimal digits) vendor code assigned by IEEE.

Device identifier: The device identifier is a unique 24-bit (six hexadecimal digits) value within a common OUI.

An EUI-64 Interface ID is represented in binary and is made up of three parts:

24-bit OUI from the client MAC address, but the 7th bit (the Universally/Locally [U/L] bit) is reversed. This means that if the 7th bit is a 0. it becomes a 1, and vice versa.

The inserted 16-bit value FFFE (in hexadecimal).

24-bit Device Identifier from the client MAC address.

The EUI-64 process is described next and illustrated in Figure 7-75, using R1’s Gigabit Ethernet MAC address of FC99:4775:CEE0.

Figure 7-75 EUI-64 Process

Step 1. Divide the MAC address between the OUI and device identifier.

Step 2. Insert the hexadecimal value FFFE, which in binary is 1111 1111 1111 1110.

Step 3. Convert the first two hexadecimal values of the OUI to binary and flip the U/L bit (bit 7). In this example, the 0 in bit 7 is changed to a 1.

The result is an EUI-64-generated Interface ID of FE99:47FF:FE75:CEE0.

Note

The use of the U/L bit and the reasons for reversing its value are discussed in RFC 5342.

The advantage of EUI-64 is that the Ethernet MAC address can be used to determine the Interface ID. It also allows network administrators to easily track an IPv6 address to an end device by using the unique MAC address. However, this has caused privacy concerns among many users. They are concerned that their packets can be traced to the actual physical computer. Due to these concerns, a randomly generated Interface ID may be used instead.

Randomly Generated Interface IDs

Depending upon the operating system, a device may use a randomly generated Interface ID instead of using the MAC address and the EUI-64 process. For example, beginning with Windows Vista, Windows uses a randomly generated Interface ID instead of one created with EUI-64. Windows XP and previous Windows operating systems used EUI-64.

An easy way to identify that an address was more than likely created using EUI-64 is the FFFE located in the middle of the Interface ID.

Create a Global Unicast or Link-Local Address

After the Interface ID is established, either through the EUI-64 process or through random generation, it can be combined with an IPv6 prefix to create a global unicast address or a link-local address:

Global unicast address: When using SLAAC, the device receives its prefix from the ICMPv6 RA and combines it with the Interface ID.

Link-local address: A link-local prefix begins with FE80::/10. A device typically uses FE80::/64 as the prefix/prefix length, followed by the Interface ID.

Dynamic Link-Local Addresses (7.2.4.6)

When using SLAAC (SLAAC Only or SLAAC with DHCPV6), a device receives its prefix and prefix length from the ICMPv6 RA. Because the prefix of the address has been designated by the RA message, the device must provide only the Interface ID portion of its address, as shown in Figure 7-76. As stated previously, the Interface ID can be automatically generated using the EUI-64 process or, depending on the OS, randomly generated. Using the information from the RA message and the Interface ID, the device can establish its global unicast address.

Figure 7-76 IPv6 Link-Local Address

After a global unicast address is assigned to an interface, the IPv6-enabled device will automatically generate its link-local address. IPv6-enabled devices must have, at a minimum, the link-local address. Recall that an IPv6 link-local address enables a device to communicate with other IPv6-enabled devices on the same subnet.

Note

An IPv6 PC will create the link-local address before it enables the IPv6 global address.

IPv6 link-local addresses are used for a variety of purposes, including

A host uses the link-local address of the local router for its default gateway IPv6 address.

Routers exchange dynamic routing protocol messages using link-local addresses.

Routers’ routing tables use the link-local address to identify the next-hop router when forwarding IPv6 packets.

A link-local address can be established dynamically or configured manually as a static link-local address.

Dynamically Assigned Link-Local Address

The link-local address is dynamically created using the FE80::/10 prefix and the Interface ID. By default, Cisco IOS routers use EUI-64 to generate the Interface ID for all link-local addresses on IPv6 interfaces. For serial interfaces, the router will use the MAC address of an Ethernet interface. Recall that a link-local address must be unique only on that link or network. However, a drawback to using the dynamically assigned link-local address is its length, which makes identifying and remembering assigned addresses challenging.

Static Link-Local Addresses (7.2.4.7)

This section explains how to manually create a link-local address on a router interface.

Static Link-Local Address

Configuring the link-local address manually provides the ability to create an address that is recognizable and easier to remember. Link-local addresses can be configured manually using the same interface command used to create IPv6 global unicast addresses but with an additional parameter:

Router(config-if)# ipv6 address link-local-address link-local

Figure 7-77 shows that a link-local address has a prefix within the range FE80 to FEBF. When an address begins with this hextet (16-bit segment), the link-local parameter must follow the address.

Figure 7-77 Configuring Link-Local Addresses on R1

Figure 7-78 shows the configuration of a link-local address using the ipv6 address interface command. The link-local address FE80::1 is used to make it easily recognizable as belonging to router R1. The same IPv6 link-local address is configured on all of R1’s interfaces. FE80::1 can be configured on each link because it only has to be unique on that link.

Figure 7-78 Configuring Link-Local Addresses on R1 (show Command)

Similar to R1, router R2 would be configured with FE80::2 as the IPv6 link-local address on all of its interfaces.

Verifying IPv6 Address Configuration (7.2.4.8)

As shown in Figure 7-79, the command to verify the IPv6 interface configuration is similar to the command used for IPv4.

Figure 7-79 show ipv6 interface brief Command

The show interface command displays the MAC address of the Ethernet interfaces. EUI-64 uses this MAC address to generate the Interface ID for the link-local address. Additionally, the show ipv6 interface brief command displays abbreviated output for each of the interfaces. The [up/up] output on the same line as the interface indicates the Layer 1/Layer 2 interface state. This is the same as the Status and Protocol columns in the equivalent IPv4 command.

Notice that each interface has two IPv6 addresses. The second address for each interface is the global unicast address that was configured. The first address, the one that begins with FE80, is the link-local unicast address for the interface. Recall that the link-local address is automatically added to the interface when a global unicast address is assigned.

Also, notice that R1’s Serial 0/0/0 link-local address is the same as its Gigabit Ethernet 0/0 interface. Serial interfaces do not have an Ethernet MAC address, so Cisco IOS uses the MAC address of the first available Ethernet interface. This is possible because link-local interfaces only have to be unique on that link.

The link-local address of the router interface is typically the default gateway address for devices on that link or network.

As shown in Figure 7-80, the show ipv6 route command can be used to verify that IPv6 networks and specific IPv6 interface addresses have been installed in the IPv6 routing table. The show ipv6 route command will only display IPv6 networks, not IPv4 networks.

Figure 7-80 show ipv6 route Command

Within the route table, a C next to a route indicates that this is a directly connected network. When the router interface is configured with a global unicast address and is in the “up/up” state, the IPv6 prefix and prefix length is added to the IPv6 routing table as a connected route.

The IPv6 global unicast address configured on the interface is also installed in the routing table as a local route. The local route has a /128 prefix. Local routes are used by the routing table to efficiently process packets with a destination address of the router’s interface address.

The ping command for IPv6 is identical to the command used with IPv4, except that an IPv6 address is used. In the example shown in Figure 7-81, the command is used to verify Layer 3 connectivity between R1 and PC1. When pinging a link-local address from a router, Cisco IOS will prompt the user for the exit interface. Because the destination link-local address can be on one or more of its links or networks, the router needs to know to which interface to send the ping.

Figure 7-81 ping Command

Activity 7.2.4.8: Verifying IPv6 Address Configuration

Go to the online course to use the Syntax Checker to verify IPv6 address configuration

IPv6 Multicast Addresses (7.2.5)

IPv6 multicast addresses are similar to IPv4 multicast addresses. Recall that a multicast address is used to send a single packet to one or more destinations (multicast group). IPv6 multicast addresses have the prefix FF00::/8.

Note

Multicast addresses can be destination addresses only, not source addresses.

There are two types of IPv6 multicast addresses:

Assigned multicast

Solicited-node multicast

Assigned Multicast

Assigned multicast addresses are reserved multicast addresses for predefined groups of devices. An assigned multicast address is a single address used to reach a group of devices running a common protocol or service. Assigned multicast addresses are used in context with specific protocols such as DHCPv6.

Two common IPv6 assigned multicast groups are

FF02::1 All-nodes multicast group: This is a multicast group that all IPv6-enabled devices join. A packet sent to this group is received and processed by all IPv6 interfaces on the link or network. This has the same effect as a broadcast address in IPv4. Figure 7-82 shows an example of communication using the all-nodes multicast address. An IPv6 router sends ICMPv6 RA messages to the all-nodes multicast group. The RA message informs all IPv6-enabled devices on the network about addressing information, such as the prefix, prefix length, and default gateway.

Figure 7-82 IPv6 All-Nodes Multicast Communications

FF02::2 All-routers multicast group: This is a multicast group that all IPv6 routers join. A router becomes a member of this group when it is enabled as an IPv6 router with the ipv6 unicast-routing global configuration command. A packet sent to this group is received and processed by all IPv6 routers on the link or network.

IPv6-enabled devices send ICMPv6 RS messages to the all-routers multicast address. The RS message requests an RA message from the IPv6 router to assist the device in its address configuration.

Solicited-Node IPv6 Multicast Addresses (7.2.5.2)

A solicited-node multicast address is similar to the all-nodes multicast address. Recall that the all-nodes multicast address is essentially the same thing as an IPv4 broadcast. All devices on the network must process traffic sent to the all-nodes address. To reduce the number of devices that must process traffic, use a solicited-node multicast address.

A solicited-node multicast address is an address that matches only the last 24 bits of the IPv6 global unicast address of a device. The only devices that need to process these packets are those devices that have these same 24 bits in the least significant, far-right portion of their Interface ID, as shown in Figure 7-83.

Figure 7-83 IPv6 Solicited-Node Multicast Address

An IPv6 solicited-node multicast address is automatically created when the global unicast or link-local unicast addresses are assigned. The IPv6 solicited-node multicast address is created by combining a special FF02:0:0:0:0:FF00::/104 prefix with the rightmost 24 bits of its unicast address.

The solicited-node multicast address consists of two parts:

FF02:0:0:0:0:1:FF00::/104 multicast prefix: This is the first 104 bits of the solicited-node multicast address.

Least significant 24 bits: These are the last or rightmost 24 bits of the solicited-node multicast address. These bits are copied from the rightmost 24 bits of the global unicast or link-local unicast address of the device.

It is possible that multiple devices will have the same solicited-node multicast address. Although rare, this can occur when devices have the same rightmost 24 bits in their Interface IDs. This does not create any problems because the device will still process the encapsulated message, which will include the complete IPv6 address of the device in question.

Packet Tracer Activity 7.2.5.3: Configuring IPv6 Addressing

In this activity you will practice configuring IPv6 addresses on a router, servers, and clients. You will also practice verifying your IPv6 addressing implementation.

Lab 7.2.5.4: Identifying IPv6 Addresses

In this lab you will identify different types of IPv6 addresses, examine a host IPv6 Network Interface and address, practice IPv6 address abbreviations, and identify the IPv6 global unicast address network prefix.

Lab 7.2.5.5: Configuring IPv6 Addresses on Network Devices

In this lab you will set up the topology and configure basic router and switch setting applying IPv6 addresses and verifying network connectivity.

Connectivity Verification (7.3)

This section looks at the different tools available to verify connectivity.

ICMP (7.3.1)

One of the tools available to verify connectivity is the Internet Control Message Protocol.

ICMPv4 and ICMPv6 Messages (7.3.1.1)

Although IP is not a reliable protocol, the TCP/IP suite does provide for four messages to be sent in the event of certain errors. These messages are sent using the services of ICMP. The purpose of these messages is to provide feedback about issues related to the processing of IP packets under certain conditions, not to make IP reliable. ICMP messages are not required, and often they are not allowed within a network for security reasons.

ICMP is available for both IPv4 and IPv6. ICMPv4 is the messaging protocol for IPv4. ICMPv6 provides these same services for IPv6 but includes additional functionality. In this course, the term ICMP will be used when referring to both ICMPv4 and ICMPv6.

The types of ICMP messages, and the reasons why they are sent, are extensive. We will discuss some of the more common messages.

ICMP messages common to both ICMPv4 and ICMPv6 include

Host Confirmation

Destination or Service Unreachable

Time Exceeded

Route Redirection

Host Confirmation

ICMP Echo messages can be used to determine if a host is operational. The local host sends an ICMP Echo Request to a host. If the host is available, the destination host responds with an Echo Reply. This use of the ICMP Echo messages is the basis of the ping utility.

Video 7.3.1.1:

View this video in the online course for a demonstration of the ICMP Echo Request/Echo Reply message. In the video, click the Play button to see an animation of the ICMP Echo Request/Echo Reply.

Destination or Service Unreachable

When a host or gateway receives a packet that it cannot deliver, it can use an ICMP Destination Unreachable message to notify the source that the destination or service is unreachable. The message will include a code that indicates why the packet could not be delivered.

Some of the Destination Unreachable codes for ICMPv4 are

0: Net Unreachable

1: Host Unreachable

2: Protocol Unreachable

3: Port Unreachable

Note

ICMPv6 has similar but slightly different codes for Destination Unreachable messages.

Time Exceeded

An ICMPv4 Time Exceeded message is used by a router to indicate that a packet cannot be forwarded because the TTL field of the packet was decremented to 0. If a router receives a packet and decrements the TTL field in the IPv4 packet to 0, it discards the packet and sends a Time Exceeded message to the source host.

ICMPv6 also sends a Time Exceeded message if the router cannot forward an IPv6 packet because the packet has expired. IPv6 does not have a TTL field; it uses the Hop Limit field to determine if the packet has expired.

Route Redirection

A router may use the ICMP Route Redirection message to notify the hosts on a network that a better route is available for a particular destination. This message may only be used when the source host is on the same physical network as both gateways.

Both ICMPv4 and ICMPv6 use Route Redirection messages.

ICMPv6 Router Solicitation and Router Advertisement Messages (7.3.1.2)

The informational and error messages found in ICMPv6 are very similar to the control and error messages implemented by ICMPv4. However, ICMPv6 has new features and improved functionality not found in ICMPv4.

ICMPv6 includes four new protocols as part of the Neighbor Discovery Protocol (ND or NDP):

Router Solicitation message (RS)

Router Advertisement message (RA)

Neighbor Solicitation message (NS)

Neighbor Advertisement message (NA)

Router Solicitation and Router Advertisement Messages

IPv6-enabled devices can be divided into two categories: routers and hosts. Router Solicitation and Router Advertisement messages are sent between hosts and routers as shown in Figure 7-84 and described here:

RS message: When a host is configured to obtain its addressing information automatically using SLAAC, the host will send an RS message to the router. The RS message is sent as an IPv6 all-routers multicast message.

RA message: RA messages are sent by routers to provide addressing information to hosts using SLAAC. The RA message can include addressing information for the host, such as the prefix and prefix length. A router will send an RA message periodically or in response to an RS message. By default, Cisco routers send RA messages every 200 seconds. RA messages are sent to the IPv6 all-nodes multicast address. A host using SLAAC will set its default gateway to the link-local address of the router that sent the RA.

Figure 7-84 Router Solicitation and Router Advertisement Messages

ICMPv6 Neighbor Solicitation and Neighbor Advertisement Messages (7.3.1.3)

ICMPv6 Neighbor Discovery Protocol includes two additional message types, Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages.

As shown in Figure 7-85, Neighbor Solicitation and Neighbor Advertisement messages are used for

Address resolution: Address resolution is used when a device on the LAN knows the IPv6 unicast address of a destination but does not know its Ethernet MAC address. To determine the MAC address for the destination, the device will send an NS message to the solicited-node multicast address. The message will include the known (targeted) IPv6 address. The device that has the targeted IPv6 address will respond with an NA message containing its Ethernet MAC address. Address resolution in IPv6 works similarly to ARP in IPv4.

Duplicate Address Detection (DAD): When a device is assigned a global unicast or link-local unicast address, performing DAD on the address is recommended to ensure that it is unique. To check the uniqueness of an address, the device sends an NS message with its own IPv6 address as the targeted IPv6 address. If another device on the network has this address, it responds with an NA message. This NA message will notify the sending device that the address is in use. If a corresponding NA message is not returned within a certain period of time, the unicast address is unique and acceptable for use, DAD works similarly to gratuitous ARP in IPv4.

Figure 7-85 ICMPv6 Neighbor Discovery Protocol

Note

DAD is not required, but RFC 4861 recommends performing DAD on unicast addresses.

Testing and Verification (7.3.2)

Several utilities are available to assist in the testing and verification of network connectivity. This section looks at two of them: ping and traceroute.

Ping: Testing the Local Stack (7.3.2.1)

Ping is a testing utility that uses ICMP Echo Request and Echo Reply messages to test connectivity between hosts. Ping works with both IPv4 and IPv6 hosts.

To test connectivity to another host on a network, an Echo Request is sent to the host address using the ping command. If the host at the specified address receives the Echo Request, it responds with an Echo Reply. As each Echo Reply is received, ping provides feedback on the length of time between when the Echo Request was sent and when the Echo Reply was received. This can be a measure of network performance.

Ping has a timeout value for the reply. If a reply is not received within the timeout, ping provides a message indicating that a response was not received. This usually indicates that there is a problem, but could also indicate that security features blocking ping messages have been enabled on the network.

After all the requests are sent, the ping utility provides a summary that includes the success rate and average round-trip time to the destination.

Pinging the Local Loopback

There are some special testing and verification cases for which we can use ping. One case is for testing the internal configuration of IPv4 or IPv6 on the local host. To perform this test, we ping the local loopback address of 127.0.0.1 for IPv4 or ::1 for IPv6. Testing the IPv4 loopback is shown in Figure 7-86.

Figure 7-86 ICMPv4 Testing Local TCP/IP Stack

A response from 127.0.0.1 for IPv4, or ::1 for IPv6, indicates that IP is properly installed on the host. This response comes from the network layer. This response is not, however, an indication that the addresses, masks, or gateways are properly configured. Nor does it indicate anything about the status of the lower layer of the network stack. This simply tests IP down through the network layer of IP. If we get an error message, it is an indication that TCP/IP is not operational on the host.

Ping: Testing Connectivity to the Local LAN (7.3.2.2)

You can also use ping to test the ability of a host to communicate on the local network. This is generally done by pinging the IP address of the gateway of the host. A successful ping to the gateway indicates that the host and the router interface serving as the gateway are both operational on the local network, as shown in Figure 7-87.

Figure 7-87 Testing IPv4 Connectivity to Local Network

For this test, the gateway address is most often used because the router is normally always operational. If the gateway address does not respond, a ping can be sent to the IP address of another host on the local network that is known to be operational.

If either the gateway or another host responds, then the local host can successfully communicate over the local network. If the gateway does not respond but another host does, this could indicate a problem with the router interface serving as the gateway.

One possibility is that the wrong gateway address has been configured on the host. Another possibility is that the router interface may be fully operational but have security applied to it that prevents it from processing or responding to ping requests.

Ping: Testing Connectivity to Remote Device (7.3.2.3)

Ping can also be used to test the ability of a local host to communicate across an internetwork. The local host can ping an operational IPv4 host of a remote network.

Video 7.3.2.3

View this video in the online course for a demonstration of the use of ping to test the ability of a local host to communicate across an internetwork.

If this ping is successful, the operation of a large piece of the internetwork can be verified. A successful ping across the internetwork confirms communication on the local network, the operation of the router serving as our gateway, and the operation of all other routers that might be in the path between the local network and the network of the remote host.

Additionally, functionality of the remote host can be verified. If the remote host could not communicate outside of its local network, it would not have responded.

Note

Many network administrators limit or prohibit the entry of ICMP messages into the corporate network; therefore, the lack of a ping response could be due to security restrictions.

Traceroute: Testing the Path (7.3.2.4)

Ping is used to test connectivity between two hosts, but it doesn’t provide information about the details of devices between the hosts. Traceroute (tracert) is a utility that generates a list of hops that were successfully reached along the path. This list can provide important verification and troubleshooting information. If the data reaches the destination, then the trace lists the interface of every router in the path between the hosts. If the data fails at some hop along the way, the address of the last router that responded to the trace can provide an indication of where the problem or security restrictions are found.

Round-Trip Time (RTT)

Using traceroute provides the round-trip time for each hop along the path and indicates if a hop fails to respond. The round-trip time is the time a packet takes to reach the remote host and for the response from the host to return. An asterisk (*) is used to indicate a lost or unreplied packet.

This information can be used to locate a problematic router in the path. When the display shows high response times or data losses from a particular hop, it is an indication that the resources of the router or its connections may be stressed.

IPv4 TTL and IPv6 Hop Limit

Traceroute makes use of a function of the TTL field in IPv4 and the Hop Limit field in IPv6 in the Layer 3 headers, along with the ICMP Time Exceeded message.

Play the animation in the video to see how traceroute takes advantage of TTL.

Video 7.3.2.4

View this video in the online course for a demonstration on the use of TTL by traceroute.

The first sequence of messages sent from traceroute will have a TTL field value of 1. This causes the TTL to time out the IPv4 packet at the first router. This router then responds with an ICMPv4 message. Traceroute now has the address of the first hop.

Traceroute then progressively increments the TTL field (2, 3, 4...) for each subsequent sequence of messages. This provides the trace with the address of each hop as the packets time out further down the path. The TTL field continues to be increased until the destination is reached or the field is incremented to a predefined maximum.

When the final destination is reached, the host responds with either an ICMP Port Unreachable message or an ICMP Echo Reply message instead of the ICMP Time Exceeded message.

Packet Tracer Activity 7.3.2.5: Verifying IPv4 and IPv6 Addressing

IPv4 and IPv6 can coexist on the same network. From the command prompt of a PC, there are some differences in the way commands are issued and in the way output is displayed. In this activity you will use ifconfig to verify both IPv4 and IPv6 addressing as well as use tracert to discover IPv4 and IPv6 paths.

Packet Tracer Activity 7.3.2.6: Pinging and Tracing to Test the Path

There are connectivity issues in this activity. In addition to gathering and documenting information about the network, you will locate the problems and implement acceptable solutions to restore connectivity.

Lab 7.3.2.7: Testing Network Connectivity with Ping and Traceroute

In this lab, you will complete the following objectives:

Part 1: Build and Configure the Network

Part 2: Use Ping Command for Basic Network Testing

Part 3: Use Tracert and Traceroute Commands for Basic Network Testing

Part 4: Troubleshoot the Topology

Packet Tracer Activity 7.3.2.8: Troubleshooting IPv4 and IPv6 Addressing

In this scenario you are a network technician working for a company that has decided to migrate from IPv4 to IPv6. In the interim, they must support both protocols (dual stack). Three co-workers have called the help desk with problems and have received limited assistance. The help desk has escalated the matter to you, a Level 2 support technician.

Summary (7.4)

Class Activity 7.4.1.1: The Internet of Everything...Naturally!

In this class activity you will complete the following objectives:

Part 1: Choose an activity

Part 2: Devise an IPv6 addressing scheme

Part 3: Present your addressing scheme to the class

Packet Tracer Activity 7.4.1.2: Skills Integration Challenge

In this scenario your company has won a contract to set up a small network for a restaurant owner. There are two restaurants near each other, and they share one connection. The equipment and cabling is installed and the network administrator has designed the implementation plan. You job is to implement the rest of the addressing scheme according to the abbreviated Addressing Table and verify connectivity.

IP addresses are hierarchical, with network, subnetwork, and host portions. An IP address can represent a complete network, a specific host, or the broadcast address of the network.

Understanding binary notation is important when determining if two hosts are in the same network. The bits within the network portion of the IP address must be identical for all devices that reside in the same network. The subnet mask or prefix length is used to determine the network portion of an IP address. IP addresses can be assigned either statically or dynamically. DHCP enables the automatic assignment of addressing information such as the IP address, subnet mask, default gateway, and other configuration information.

IPv4 hosts can communicate in one of three different ways: unicast, broadcast, or multicast. Blocks of addresses that are used in networks that require limited or no Internet access are called private addresses. The private IPv4 address blocks are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.

The depletion of IPv4 address space is the motivating factor for moving to IPv6. Each IPv6 address has 128 bits, versus the 32 bits in an IPv4 address. IPv6 does not use the dotted-decimal subnet mask notation. The prefix length is used to indicate the network portion of an IPv6 address using the following format: IPv6 address/prefix length.

There are three types of IPv6 addresses: unicast, multicast, and anycast. An IPv6 link-local address enables a device to communicate with other IPv6-enabled devices on the same link, and only on that link (subnet). Packets with a source or destination link-local address cannot be routed beyond the link from where the packet originated. IPv6 link-local addresses are in the FE80::/10 range.

ICMP is available for both IPv4 and IPv6. ICMPv4 is the messaging protocol for IPv4. ICMPv6 provides the same services for IPv6 but includes additional functionality.

After it is implemented, an IP network needs to be tested to verify its connectivity and operational performance. The ping and traceroute utilities are two tools that can be used to test network connectivity.

Practice

The following activities provide practice with the topics introduced in this chapter. The Labs and Class Activities are available in the companion Network Basics Lab Manual (978-1-58713-313-8). The Packet Tracer Activities PKA files are found in the online course.

Class Activities

Class Activity 7.0.1.2: The Internet of Everything (IoE)

Class Activity 7.4.1.1: The Internet of Everything...Naturally!

Labs

Lab 7.1.2.7: Using the Windows Calculator with Network Addresses

Lab 7.1.2.8: Converting IPv4 Addresses to Binary

Lab 7.1.4.8: Identifying IPv4 Addresses

Lab 7.2.5.4: Identifying IPv6 Addresses

Lab 7.2.5.5: Configuring IPv6 Addresses on Network Devices

Lab 7.3.2.7: Testing Network Connectivity with Ping and Traceroute

Packet Tracer Activities

Packet Tracer Activity 7.1.3.8: Investigate Unicast, Broadcast, and Multicast Traffic

Packet Tracer Activity 7.2.5.3: Configuring IPv6 Addressing

Packet Tracer Activity 7.3.2.5: Verifying IPv4 and IPv6 Addressing

Packet Tracer Activity 7.3.2.6: Pinging and Tracing to Test the Path

Packet Tracer Activity 7.3.2.8: Troubleshooting IPv4 and IPv6 Addressing

Packet Tracer Activity 7.4.1.2: Skills Integration Challenge

Check Your Understanding

Complete all the review questions listed here to test your understanding of the topics and concepts in this chapter. The appendix, “Answers to the ‘Check Your Understanding’ Questions,” lists the answers.

1. The eight-digit binary value of the last octet of the IPv4 address 172.17.10.7 is ________.

2. Which two statements describe broadcast transmissions on a wired network? (Choose two.)

A. Directed broadcasts use the IP address of 255.255.255.255.

B. Limited broadcasts are intended for local and remote hosts listening for a multicast packet.

C. Directed broadcasts are intended for all hosts on a local or remote network.

D. Limited broadcasts are only intended for all hosts on a local network.

E. Limited broadcasts are forwarded by routers.

3. What is the prefix length notation for the subnet mask 255.255.255.224?

A. /25

B. /26

C. /27

D. /28

4. The greatest part of the /8 block IPv4 address bit space consists of what types of addresses?

A. Private addresses

B. Public addresses

C. Multicast addresses

D. Experimental address

5. The most compact form of the IPv6 address 2001:0DB8:0000:AB00:0000:0000:0000:00AB is ___________________.

6. Which type of IPv6 address is not routable and is used only for communication on a single subnet?

A. Global unicast address

B. Link-local address

C. Loopback address

D. Unique local address

E. Unspecified address

7. An organization is assigned an IPv6 global prefix of 2001:db8:1234::/48. What is the maximum number of /64 subnets the organization can create with this space?

A. 4

B. 16

C. 1,024

D. 65,536

8. Match the description to the IPv6 addressing component. (Not all options are used.)

9. Which two ICMPv6 messages are not present in ICMP for IPv4?

A. Destination Unreachable

B. Host Confirmation

C. Neighbor Solicitation

D. Route Redirection

E. Router Advertisement

F. Time Exceeded

10. Which IP addresses are network addresses? (Choose two.)

A. 64.104.3.7/28

B. 192.168.12.64/26

C. 192.135.12.191/26

D. 198.18.12.16/28

E. 209.165.200.254/27

F. 220.12.12.33/27

11. A network administrator is building a network for a small business that has 22 hosts. The ISP has assigned only one Internet-routable IP address. Which IP address block can the network administrator use to address the network?

A. 10.11.12.16/28

B. 172.31.255.128/27

C. 192.168.1.0/28

D. 209.165.202.128/27

12. Which subnet mask would be used with the hosts in the 128.107.176.0/22 network?

A. 255.0.0.0

B. 255.248.0.0

C. 255.255.252.0

D. 255.255.255.0

E. 255.255.255.252

13. You have been assigned the address block 10.255.255.224/28 to create the network addresses for point-to-point wide area network (WAN) links. How many of these WAN links can you support with this address block?

A. 1

B. 4

C. 7

D. 14

14. A network administrator needs to create a new network that has 14 computers and two router interfaces. What subnet mask will provide the appropriate^oo number of IPv4 addresses for this network with minimal wasted addresses?

A. 255.255.255.128

B. 255.255.255.192

C. 255.255.255.224

D. 255.255.255.240

E. 255.255.255.248

F. 255.255.255.252

15. A host in the south branch cannot access the server with the address 192.168.254.222/224. While examining the host, you determine that its IPv4 address is 169.254.11.15/16. What is the apparent problem?

A. The host is using a link-local address.

B. The server is using an invalid subnet mask.

C. The host has been assigned a broadcast address.

D. The server thinks that the host is on the logical network with the server.

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Table of Contents for Chapter 7. IP Addressing

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