IP Addressing

One of the most important topics in any discussion of TCP/IP is IP addressing. An IP address is a numeric identifier assigned to each machine on an IP network. It designates the specific location of a device on the network.

An IP address is a software address, not a hardware address—the latter is hard-coded on a network interface card (NIC) and used for finding hosts on a local network. IP addressing was designed to allow hosts on one network to communicate with a host on a different network regardless of the type of LANs the hosts are participating in.

Before I get into the more complicated aspects of IP addressing, you need to understand some of the basics. First I'm going to explain some of the fundamentals of IP addressing and its terminology. Then you'll learn about the hierarchical IP addressing scheme and private IP addresses.

IP Terminology

In the rest of this chapter you'll learn several important terms vital to your understanding of the Internet Protocol. Here are a few to get you started:

Bit A bit is one digit, either a 1 or a 0.

Byte A byte is 7 or 8 bits, depending on whether parity is used. For the rest of this chapter, always assume a byte is 8 bits.

Octet An octet, made up of 8 bits, is just an ordinary 8-bit binary number. In this chapter, the terms byte and octet are completely interchangeable.

Network Address This is the designation used in routing to send packets to a remote network—for example, 10.0.0.0, 172.16.0.0, and 192.168.10.0.

Broadcast Address The address used by applications and hosts to send information to all nodes on a network is called the broadcast address. Examples include 255.255.255.255, which is any network, all nodes; 172.16.255.255, which is all subnets and hosts on network 172.16.0.0; and 10.255.255.255, which broadcasts to all subnets and hosts on network 10.0.0.0.

The Hierarchical IP Addressing Scheme

An IP address consists of 32 bits of information. These bits are divided into four sections, referred to as octets or bytes, each containing 1 byte (8 bits). You can depict an IP address using one of three methods:

• Dotted-decimal, as in 172.16.30.56
• Binary, as in 10101100.00010000.00011110.00111000
• Hexadecimal, as in AC.10.1E.38

All these examples truly represent the same IP address. Hexadecimal isn't used as often as dotted-decimal or binary when IP addressing is discussed, but you still might find an IP address stored in hexadecimal in some programs. The Windows Registry is a good example of a program that stores a machine's IP address in hex.

The 32-bit IP address is a structured or hierarchical address, as opposed to a flat or nonhierarchical address. Although either type of addressing scheme could have been used, hierarchical addressing was chosen for a good reason. The advantage of this scheme is that it can handle a large number of addresses, namely, 4.3 billion (a 32-bit address space with two possible values for each position—either 0 or 1—gives you 2³², or 4,294,967,296). The disadvantage of the flat addressing scheme, and the reason it's not used for IP addressing, relates to routing. If every address were unique, all routers on the Internet would need to store the address of each and every machine on the Internet. This would make efficient routing impossible, even if only a fraction of the possible addresses were used.

The solution to this problem is to use a two- or three-level hierarchical addressing scheme that is structured by network and host or by network, subnet, and host.

This two- or three-level scheme is comparable to a telephone number. The first section, the area code, designates a very large area. The second section, the prefix, narrows the scope to a local calling area. The final segment, the customer number, zooms in on the specific connection. IP addresses use the same type of layered structure. Rather than all 32 bits being treated as a unique identifier, as in flat addressing, part of the address is designated as the network address, and the other part is designated as either the subnet and host or just the node address.

In the following sections, I'll discuss IP network addressing and the different classes of address you can use to address your networks.

Network Addressing

The network address (which can also be called the network number) uniquely identifies each network. Every machine on the same network shares that network address as part of its IP address. In the IP address 172.16.30.56, for example, 172.16 is the network address.

The node address is assigned to, and uniquely identifies, each machine on a network. This part of the address must be unique because it identifies a particular machine—an individual—as opposed to a network, which is a group. This number can also be referred to as a host address. In the sample IP address 172.16.30.56, the 30.56 is the node address.

The designers of the Internet decided to create classes of networks based on network size. For the small number of networks possessing a very large number of nodes, they created the rank Class A network. At the other extreme is the Class C network, which is reserved for the numerous networks with a small number of nodes. The class distinction for networks between very large and very small is predictably called the Class B network.

Subdividing an IP address into a network and node address is determined by the class designation of one's network. Figure 2.13 summarizes the three classes of networks used to address hosts with—a subject I'll explain in much greater detail throughout the rest of this chapter.

FIGURE 2.13 Summary of the three classes of networks

To ensure efficient routing, Internet designers defined a mandate for the leading-bits section of the address for each different network class. For example, since a router knows that a Class A network address always starts with a 0, the router might be able to speed a packet on its way after reading only the first bit of its address. This is where the address schemes define the difference between a Class A, a Class B, and a Class C address. In the next sections, I'll discuss the differences between these three classes, followed by a discussion of the Class D and Class E addresses (Classes A, B, and C are the only ranges that are used to address hosts in your networks).

Network Address Range: Class A

The designers of the IP address scheme said that the first bit of the first byte in a Class A network address must always be off, or 0. This means a Class A address must be between 0 and 127 in the first byte, inclusive.

Consider the following first byte of a network address:

0xxxxxxx

If you turn the other 7 bits all off and then turn them all on, you'll find the Class A range of network addresses.

00000000 = 0
01111111 = 127

So, a Class A network is defined in the first octet between 0 and 127, and it can't be less or more. (Yes, I know 0 and 127 are not valid in a Class A network. I'll talk about reserved addresses in a minute.)

Network Address Range: Class B

In a Class B network, the RFCs state that the first bit of the first byte must always be turned on, but the second bit must always be turned off. If you turn the other 6 bits all off and then all on, you will find the range for a Class B network.

10000000 = 128
10111111 = 191

As you can see, a Class B network is defined when the first byte is configured from 128 to 191.

Network Address Range: Class C

For Class C networks, the RFCs define the first 2 bits of the first octet as always turned on, but the third bit can never be on. Following the same process used with the previous classes, convert from binary to decimal to find the range. Here's the range for a Class C network:

11000000 = 192
11011111 = 223

So, if you see an IP address that starts at 192 and goes to 223 in the first octet, you'll know it is a Class C IP address.

Network Address Ranges: Classes D and E

The addresses between 224 to 255 in the first octet are reserved for Class D and E networks. Class D (224–239) is used for multicast addresses and Class E (240–255) for scientific purposes, but I'm not going into these types of addresses in this book (and you don't need to know them).

Network Addresses: Special Purpose

Some IP addresses are reserved for special purposes, so network administrators can't ever assign these addresses to nodes. Table 2.4 lists the members of this exclusive little club and the reasons why they're included in it.

TABLE 2.4 Reserved IP addresses

Class A Addresses

In a Class A network address, the first byte is assigned to the network address, and the three remaining bytes are used for the node addresses. The Class A format is as follows:

network.node.node.node

For example, in the IP address 49.22.102.70, the 49 is the network address, and 22.102.70 is the node address. Every machine on this particular network would have the distinctive network address of 49.

Class A network addresses are 1 byte long, with the first bit of that byte reserved and the 7 remaining bits available for manipulation (addressing). As a result, the maximum number of Class A networks that can be created is 128. Why? Because each of the 7 bit positions can be either a 0 or a 1, thus 2⁷, or 128.

To complicate matters further, the network address of all 0s (00000000) is reserved to designate the default route (see Table 2.4 in the previous section). Additionally, the address 127, which is reserved for diagnostics, can't be used either, which means you can really only use the numbers 1 to 126 to designate Class A network addresses. This means the actual number of usable Class A network addresses is 128 minus 2, or 126.

Each Class A address has 3 bytes (24-bit positions) for the node address of a machine. This means there are 2²⁴—or 16,777,216—unique combinations and, therefore, precisely that many possible unique node addresses for each Class A network. Because node addresses with the two patterns of all 0s and all 1s are reserved, the actual maximum usable number of nodes for a Class A network is 2²⁴ minus 2, which equals 16,777,214. Either way, that's a huge amount of hosts on a network segment!

Class A Valid Host IDs

Here's an example of how to figure out the valid host IDs in a Class A network address:

• All host bits off is the network address: 10.0.0.0.
• All host bits on is the broadcast address: 10.255.255.255.

The valid hosts are the numbers in between the network address and the broadcast address: 10.0.0.1 through 10.255.255.254. Notice that with the exception of the two IP addresses in the bullets above, 0s and 255s can be used as valid host ID values. All you need to remember when trying to find valid host addresses is that the host bits can't all be turned off or all be on at the same time.

Class B Addresses

In a Class B network address, the first 2 bytes are assigned to the network address, and the remaining 2 bytes are used for node addresses. The format is as follows:

network.network.node.node

For example, in the IP address 172.16.30.56, the network address is 172.16, and the node address is 30.56.

With a network address being 2 bytes (8 bits each), there would be 2¹⁶ unique combinations. But the Internet designers decided that all Class B network addresses should start with the binary digit 1 and then 0. This leaves 14 bit positions to manipulate, which is 16,384 (that is, 2¹⁴) unique Class B network addresses.

A Class B address uses 2 bytes for node addresses. This is 2¹⁶ minus the two reserved patterns (all 0s and all 1s), for a total of 65,534 possible node addresses for each Class B network.

Class B Valid Host IDs

Here's an example of how to find the valid hosts in a Class B network:

• All host bits turned off is the network address: 172.16.0.0.
• All host bits turned on is the broadcast address: 172.16.255.255.

The valid hosts would be the numbers in between the network address and the broadcast address: 172.16.0.1 through 172.16.255.254.

Class C Addresses

The first 3 bytes of a Class C network address are dedicated to the network portion of the address, with only 1 measly byte remaining for the node address. Here's the format:

network.network.network.node

Using the example IP address 192.168.100.102, the network address is 192.168.100, and the node address is 102.

In a Class C network address, the first three bit positions are always the binary 110. The calculation is as follows: 3 bytes, or 24 bits, minus 3 reserved positions leaves 21 positions. Hence, there are 2²¹, or 2,097,152, possible Class C networks.

Each unique Class C network has 1 byte to use for node addresses. This leads to 2⁸ or 256, minus the two reserved patterns of all 0s and all 1s, for a total of 254 node addresses for each Class C network.

Class C Valid Host IDs

Here's an example of how to find a valid host ID in a Class C network:

• All host bits turned off is the network ID: 192.168.100.0.
• All host bits turned on is the broadcast address: 192.168.100.255.

The valid hosts would be the numbers in between the network address and the broadcast address: 192.168.100.1 through 192.168.100.254.

Private IP Addresses

The people who created the IP addressing scheme also created what we call private IP addresses. These addresses can be used on a private network, but they're not routable through the Internet. This is designed for the purpose of creating a measure of well-needed security, but it also conveniently saves valuable IP address space.

If every host on every network had to have real routable IP addresses, we would have run out of IP addresses to hand out years ago. But by using private IP addresses, ISPs, corporations, and home users need only a relatively tiny group of bona fide IP addresses to connect their networks to the Internet. This is economical because they can use private IP addresses on their inside networks and get along just fine.

To accomplish this task, the ISP and the corporation—the end user, no matter who they are—need to use something called Network Address Translation (NAT), which basically takes a private IP address and converts it for use on the Internet. (Chapter 3 includes an introduction to NAT.) Many people can use the same real IP address to transmit out onto the Internet. Doing things this way saves megatons of address space—good for us all!

Table 2.5 lists the reserved private addresses.

TABLE 2.5 Reserved IP address space

According to Cisco, private IP addresses are used for the following reasons:

• To create addresses that cannot be routed through the public Internet
• To conserve public addresses