## IP Addressing Review

Chapter 5 explained the concepts behind IP addressing; Class A, B, and C networks; and subnetting. Before looking at the math behind IP addressing, a quick review will be helpful.

Many different Class A, B, and C networks exist. Table 12-2 summarizes the possible network numbers, the total number of each type, and the number of hosts in each Class A, B, and C network.

 Class A Class B Class C First Octet Range 1 to 126 128 to 191 192 to 223 Valid Network Numbers 1.0.0.0 to 126.0.0.0 128.1.0.0 to 191.254.0.0 192.0.1.0 to 223.255.254.0 Number of Networks of This Class 27 - 2 214 - 2 221 - 2 Number of Hosts per Network 224 - 2 216 - 2 28 - 2 Size of Network Part of Address (bytes) 1 2 3 Size of Host Part of Address (bytes) 3 2 1

*The "Valid Network Numbers" row shows actual network numbers. There are several reserved cases. For example, networks 0.0.0.0 (originally defined for use as a broadcast address) and 127.0.0.0 (still available for use as the loopback address) are reserved. Networks 128.0.0.0, 191.255.0.0, 192.0.0.0, and 223.255.255.0 also are reserved.

*The "Valid Network Numbers" row shows actual network numbers. There are several reserved cases. For example, networks 0.0.0.0 (originally defined for use as a broadcast address) and 127.0.0.0 (still available for use as the loopback address) are reserved. Networks 128.0.0.0, 191.255.0.0, 192.0.0.0, and 223.255.255.0 also are reserved.

Without subnetting, a different IP network must be used for each physical network. For example, Figure 12-1 shows three example IP addresses, each from a different network. One address is in a Class A network, one is in a Class B network, and one is in a Class C network.

Figure 12-1 Example Class A, B, and C IP Addresses and Their Formats

 Class A Network (8) Host (24) 8 1 4 5 Class B Network (16) Host (16) 130 4 100 1 Class C Network (24) Host (8) 199 1 1 . 1

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 1-byte host part, Class B has a 2-byte host part, and Class C has a 1-byte host part.