Topic: Configure, verify, and troubleshoot IPv4 addressing and subnetting

An IPv4 address is a 32-bit number that we like to represent in dotted-decimal notation. Consider using a conversion chart for the 8 bits that exist in an octet to help with the various subnetting exercises you might encounter on the CCNA 200-301exam. Figure 5.1 is a simple chart you can jot down on scratch paper before starting the exam.

One task that is simple using this chart is converting a number from decimal to binary or vice versa. For example, to convert 186 to binary, you first note that you can successfully subtract 128 from this number, so you can set the first bit to on (1). The remainder after this subtraction is 58, and you cannot subtract 64 from this number (without having a negative number), so you set the 64 value to off (0) and move to the next number. You then subtract 32 from 58, place a 1 in the 32 column, and end up with 26 as the remainder. You can subtract 16 from 26, so you put a 1 in that column. Continuing with this method, you can easily calculate that 186 in binary is 10111010.

Converting from binary to decimal is even easier: Just examine what bit positions are on (1) and add those decimal values together. So, for example, 11101111 equals 239.

Early on in the development of TCP/IP, the designers created address classes to attempt to accommodate networks of various sizes. They did this by setting the initial bit values. Table 5.1 shows these classes.

Another critical memorization point here is the default subnet masks for the address classes. Remember that it is the job of a subnet mask to define what part of a 32-bit address represents the network portion and what part represents the host portion. Table 5.2 defines the classful masks that have been defined.

Note that subnet masks must use continuous on bits (1), starting from the left (MSB). The subnet mask octet 11100000 is valid, whereas 00111111 or 11100011 are not. This results in the only possible values in a subnet mask octet being the ones shown in Table 5.3.

Remember that subnetting is a process of “stealing” or “borrowing” bits from the host portion of a classful IPv4 address in order to create additional subnets. Think of using the following IP address and subnet mask combination in your network:

1. 10.0.0.0/8 or 10.0.0.0 255.0.0.0

This allows you to create only one giant network. Sure, this network can have many host systems (specifically, 2²⁴ – 2), but they all must exist in the same network. With broadcast traffic and other potential issues, this would be terrible for efficient communications. Today, we like to divide networks into small sections (subnetworks) of about 100 computers or fewer.

In the preceding example, say that you decide to borrow 4 bits for subnetting. Now the identifications look like this:

1. 10.0.0.0 255.240.0.0 or 10.0.0.0/12

How many bits are left for host identification? The subnet mask now contains 12 bits, leaving 20 bits available for host identification. This calculation, 2²⁰ – 2, requires a calculator. As a result, you would not see this question on the CCNA 200-301 exam. (The answer, by the way, is an astounding 1,048,574 hosts per subnet.)

Another important skill you need is to be able to establish the exact subnets to create, given a bit-borrowing scenario. The great news is that you can once again rely on Figure 5.1 for assistance!

Using the preceding scenario, you have:

1. 10.0.0.0 255.240.0.0 or 10.0.0.0/12

To determine the subnets, you determine the block size by determining the least significant bit (rightmost) decimal value that the subnet mask extends to. So in this example, you extend 4 bits into the second octet. In this case, you can use Figure 5.1 to determine that the decimal value (block size) here is 16. Therefore, you start at 0, and then each new subnet increments by 16—so you have subnets numbered 0, 16, 32, 48, 64, 80, and so on. Plugging these values into the IP address given, you have the following subnets:

1. 10.0.0.0/12

2. 10.16.0.0/12

3. 10.32.0.0/12

4. 10.48.0.0/12

5. 10.64.0.0/12

6. 10.80.0.0/12

7. and so on.

What if you begin with 10.46.0.0/16 and want to borrow 4 additional bits to create new subnets? No problem. Now this is what you have:

1. 10.46.0.0/20

2. 10.46.16.0/20

3. 10.46.32.0/20

4. 10.46.48.0/20

5. 10.46.64.0/20

6. 10.46.80.0/20

7. and so on.

What if you begin with 192.168.1.0/24 and need to create six subnets? Borrowing 3 bits does the job, with some to spare (2³ = 8), and you have these subnets:

1. 192.168.1.0/27

2. 192.168.1.32/27

3. 192.168.1.64/27

4. 192.168.1.96/27

5. 192.168.1.128/27

6. 192.168.1.160/27

There are two more subnets, of course, but you do not care in this case because you need only six.

What about usable addresses for hosts on a subnet? Look at 192.168.1.0/27 above. That is a reserved address—the subnet ID itself. Add 1 to this address, and you have the first usable host address on this subnet: 192.168.1.1/27. The last address before you get to the next subnet is 192.168.1.31/27. This is reserved for the subnet broadcast. Remember from the earlier discussion that these two reserved addresses are why we subtract 2 in the formula for calculating the number of hosts. Because the last usable address is always the next subnet ID minus 2, the last usable address on the subnet is 192.168.1.30/27.

Here is one more example. If you have 10.10.0.0/16 and want at least 15 new subnets, you can create the scheme 10.10.0.0/20. Here are the usable host address ranges for the first four subnets:

1. Subnet 10.10.0.0/20: 10.10.0.1 (first usable)–10.10.15.254 (last usable)

2. Subnet 10.10.16.0/20: 10.10.16.1 (first usable)–10.10.31.254 (last usable)

3. Subnet 10.10.32.0/20: 10.10.32.1 (first usable)–10.10.47.254 (last usable)

4. Subnet 10.10.48.0/20: 10.10.48.1 (first usable)–10.10.63.254 (last usable)

Topic: Compare and contrast IPv4 address types

Modern networking systems use three main forms of IPv4 addressing for communicate in the network:

• ▸ Unicast

• ▸ Broadcast

• ▸ Multicast

Unicast transmission is most likely what you think of first. Say that you are in a home network with IP address 192.168.1.2, and you want to send data to print to a printer located at 192.168.1.10. You do not intend for any other system to receive this traffic. This is a classic example of unicast IPv4 traffic.

When you have a system that must send a frame to all members of the network, this is termed a broadcast. At Layer 2, the destination broadcast address is FF:FF:FF:FF:FF:FF.

At Layer 3, an example of a broadcast IPv4 address is 255.255.255.255. Remember that there is another type of broadcast, however, that is sent when a packet is destined for all of the members of a subnet—known as a directed broadcast, or targeted broadcast. For example, the broadcast address for subnet 10.10.0.0/20 is 10.10.15.255. (Earlier in this chapter, you calculated the broadcast addresses for subnets.)

What if you want a device to “tune into” traffic in much the same way you tune into a television station in order to enjoy a broadcast of some show? The network equivalent of this is multicasting. Remember that the multicast address range is 224–239 in the first octet. Computers can “subscribe to” or “join” a multicast group by participating in (listening to) this address scheme (in addition to their unicast address). Multicast is a way of sending packets to multiple hosts across multiple networks and subnetworks. Some routing protocols use multicast addressing. When you enable RIPv2 on a router, it starts listening for traffic destined for its 224.0.0.9 address as this is the address used to send traffic to all RIPv2 routers.

Topic: Describe the need for private IPv4 addressing

The designers of IPv4 created the private address space to help alleviate the problem of IPv4 addresses being depleted. The private address space is not routed on the public Internet but can be used as needed inside private networks. Thanks to private address space, many different companies can be using exactly the same IP address space without causing things to break badly. This address space can then be translated using Network Address Translation (NAT) to allow access to and through the public Internet.

While it is your responsibility to set up your network correctly and not send the RFC 1918 prefixes to the Internet (via your ISP), understand that ISPs are not in the business of trusting the skills of network architects. ISPs put filters on interfaces that connect to private networks. These filters drop the RFC 1918 advertisements and traffic if they inadvertently appear. It is also interesting to note that these filters filter other addresses as well. These include special use and other addresses that would not make sense to source from a private network. Packets that should not appear on the Internet are called Martian packets or bogons.

Because of private addresses and NAT, you tend to see the same address ranges (typically 192.168.1.X) used in many homes today. Table 5.4 identifies the ranges in the private address space.

Topic: Verify IP parameters for client OS (Windows, Mac OS, Linux)

Regardless of what client OS you use, there is an easy way to verify the IP parameters on network interfaces (unless the OS is administratively locked down to not allow such actions). In fact, in Chapter 6, “Configure IPv6,” you will discover that it is also as easy to do this for IPv6 information assigned to interfaces.

Example 5.1 shows the use of ipconfig on a Windows 10 system. As you can see, the output of this command provides you with basic configuration parameters. In Chapter 6, you will learn how to obtain even more information by using the /all switch after this command.

Example 5.1 Using ipconfig on a Windows 10 System

Click here to view code image

C:Users	erry>ipconfig
Windows IP Configuration
...
Wireless LAN adapter Wi-Fi:
   Connection-specific DNS Suffix  . :
   IPv6 Address. . . . . . . . . . . : 2603:9000:e70b:5300:340d:2223:70a:4585
   Temporary IPv6 Address. . . . . . : 2603:9000:e70b:5300:44b7:970b:a94b:be24
   Temporary IPv6 Address. . . . . . : 2603:9000:e70b:5300:758e:acd4:c9f0:d24d
   Link-local IPv6 Address . . . . . : fe80::340d:2223:70a:4585%5
   IPv4 Address. . . . . . . . . . . : 192.168.0.8
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Default Gateway . . . . . . . . . : fe80::92c7:92ff:fecc:dda7%5
                                       192.168.0.1

Keep in mind that you can also obtain this information in the GUI. In Windows 10, you find this information in the GUI by going to Settings, clicking the Network & Internet option, and clicking View your network properties. Figure 5.2 shows an example of this approach.

Review Questions

1. What is 2 raised to the 7th power?

A. 64

B. 128

C. 32

D. 16

2. Which of the following is true of the IP address 127.0.0.1?

A. This is a multicast address.

B. This is a Class A unicast address.

C. This is a loopback address.

D. This is an invalid IP address.

3. What is the subnet mask if you begin with the default Class A mask and then “borrow” 4 bits for subnetting?

A. 255.255.128.0

B. 255.255.240.0

C. 255.240.0.0

D. 255.255.255.240

4. If you need to create six subnets and want to waste as little IP address space as possible, how many bits should you “borrow”?

A. 2

B. 3

C. 4

D. 5

5. Examine the following diagram. What is the most likely reason Host A is unable to ping Host B?

A. The subnet masks are incorrect for the link between R1 and R2.

B. Host A has an invalid IP address.

C. Host B is attempting to use the subnet ID as an IP address.

D. The R2 interface to R1 is attempting to use a subnet broadcast IP address.

6. What is the Layer 3 broadcast address?

A. 127.255.255.255

B. 0.0.0.0

C. 1.1.1.1

D. 255.255.255.255

7. What is the range of Class B private addresses?

A. 172.16.0.0 to 172.16.255.255

B. 172.0.0.0 to 172.255.255.255

C. 172.16.0.0 to 172.31.255.255

D. 172.32.0.0 to 172.36.255.255

8. What parameters is your engineer most likely verifying when she enters ifconfig on your Linux system?

A. Duration of the current interface state

B. OS version information

C. Registry settings

D. IP address settings

Answers to Review Questions

1. B is correct. As shown on the quick reference sheet from Figure 5.1, 2⁷ = 128.

2. C is correct. 127.0.0.1 is a loopback address.

3. C is correct. The default Class A subnet mask is 255.0.0.0. Borrowing 4 bits from the next octet creates the new mask 255.240.0.0.

4. B is correct. Borrowing 3 bits makes it possible to create eight subnets. Borrowing fewer bits does not allow for as many subnets, and borrowing more than 3 bits wastes addressing space because there will be more unused subnets than when borrowing 3 bits.

5. C is correct. The Host B IP address is the subnet identifier for that subnet and is reserved.

6. D is correct. The IPv4 broadcast address is simply 255.255.255.255.

7. C is correct. The RFC 1918 /20 range is 172.16.0.0 to 172.31.255.255 for Class B networks.

8. D is correct. The ifconfig command on Linux displays IP address information.

Hands-On Lab Practice Assignment

Additional Resources

Subnetting—“I Feel the Need, the Need for Speed”

https://www.ajsnetworking.com/subnetting3

Subnetting Quiz

http://bit.ly/subnetquiz