IP address representation and classes

IP addressing uses a 32-bit address to identify each host on an IPv4 network. To make addresses easier to read, they are written in dotted decimal notation, each address being four octets in length. For example, address 00001010000000010000000100000001 in binary is written as 10.1.1.1.

Each IP address breaks down into the following sections:

IP addresses are divided into five classes, as shown in Figure 9. The shaded areas represent the address class. The first three classes are most commonly used.

Figure 9: IP address classes

Table 1: IP address classes and ranges

Class

Address range

Remarks

A

0.0.0.0 to 127.255.255.255

The IP address 0.0.0.0 is used by a host at startup for temporary communication. This address is never a valid destination address.

Addresses starting with 127 are reserved for loopback test. Packets destined to these addresses are processed locally as input packets rather than sent to the link.

B

128.0.0.0 to 191.255.255.255

N/A

C

192.0.0.0 to 223.255.255.255

N/A

D

224.0.0.0 to 239.255.255.255

Multicast addresses.

E

240.0.0.0 to 255.255.255.255

Reserved for future use, except for the broadcast address 255.255.255.255.