IP addressing

An IP version 4 address consists of 32 bits expressed in dotted-decimal format (x.x.x.x). The IP version 4 address space is divided into classes, with classes A, B, and C reserved for unicast addresses and accounting for 87.5 percent of the 32-bit IP address space. Class D is reserved for multicast addressing. The following table lists the breakdown of IP address space by address range and mask.

Class

Address range

Mask

Number of addresses

A

1.0.0.0 to 126.0.0.0

255.0.0.0

126

B

128.0.0.0 to 191.0.0.0

255.255.0.0

127 * 255

C

192.0.0.0 to 223.0.0.0

255.255.255.0

31 * 255 * 255

D

224.0.0.0 to 239.0.0.0

To express an IP address in dotted-decimal notation, you convert each octet of the IP address to a decimal number and separate the numbers by decimal points. For example, you specify the 32-bit IP address 10000000 00100000 00001010 10100111 in dotted-decimal notation as 128.32.10.167.

Each IP address class, when expressed in binary, has a different boundary point between the network and host portions of the address as illustrated in the following figure. The network portion is a network number field from 8 through 24 bits. The remaining 8 through 24 bits identify a specific host on the network.

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Network and host boundaries in IP address classes