Foundation Topics

IP Addressing Review 103

Figure 4-1 Class A, B, and C IP Addresses and Their Formats

By definition, an IP address that begins with 8 in the first octet is in a Class A network, so the network part of the address is the first byte, or first octet. An address that begins with 130 is in a Class B network. By definition, Class B addresses have a 2-byte network part, as shown. Finally, any address that begins with 199 is in a Class C network, which has a 3-byte network part. Also by definition, a Class A address has a 3-byte host part, Class B has a 2- byte host part, and Class C has a 1-byte host part.

Humans can simply remember the numbers in Table 4-2 and the concepts in Figure 4-1 and then quickly determine the network and host part of an IP address. Computers, however, use a mask to define the size of the network and the host parts of an address. The logic behind the mask results in the same conventions of Class A, B, and C networks that you already know, but the computer can deal with it better as a binary math problem. The mask is a 32bit binary number, usually written in dotted-decimal format. The purpose of the mask is to define the structure of an IP address. In short, the mask defines the size of the host parts of an IP address, representing the host part of the IP address with binary 0s in the mask. The Class A mask has its last 24 bits as binary 0, which means that the last three octets of the mask are 0s. Table 4-3 summarizes the default masks and reflects the sizes of the two parts of an IP address.

Table 4-3 Class A, B, and C Networks: Network and Host Parts and Default Masks

104 Chapter 4: IP Addressing and Subnetting

IP Subnetting

IP subnetting creates vastly larger numbers of smaller groups of IP addresses compared with simply using Class A, B, and C conventions. The Class A, B, and C rules still exist, but now a single Class A, B, or C network can be subdivided into many smaller groups. Subnetting treats a subdivision of a single Class A, B, or C network as if it were a network itself. By doing so, a single Class A, B, or C network can be subdivided into many nonoverlapping subnets.

Figures 4-2 and 4-3 show the basic differences between a network that does not use subnetting and one that does. First, look at Figure 4-2, which uses six different IP networks.

Figure 4-2 Network Topology Using Six IP Networks

Frame

Relay 150.5.0.0

D

C

150.6.0.0

150.3.0.0

150.4.0.0

Vinnie

Kris Wendell

The design shown in Figure 4-2 requires six groups, each of which is a Class B network. The four LANs each use a single Class B network. In other words, the LANs attached to Routers A, B, C, and D are each a separate network. Additionally, the two serial interfaces comprising the point-to-point serial link between Routers C and D use the same network, because these two interfaces are not separated by a router. Finally, the three router interfaces comprising

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the Frame Relay network with Routers A, B, and C are not separated by an IP router and would comprise the sixth network.

NOTE Other Frame Relay IP addressing options would require one or two more IP network numbers for this physical network.

As in Figure 4-2, the design shown in Figure 4-3 requires six groups. Unlike Figure 4-2, Figure 4-3 uses six subnets, each of which is a subnet of a single Class B network.

Figure 4-3 Same Network Topology Using One IP Network with Six Subnets

150.150.4.2

This design subnets Class B network 150.150.0.0. The IP network designer has chosen a mask of 255.255.255.0, the last octet of which implies 8 host bits. Because it is a Class B network, there are 16 network bits. Therefore, there are 8 subnet bits, which happen to be bits 17 through 24—in other words, the third octet.