IP address classes
IP addressing uses a 32-bit address to identify each host on a 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 two parts:
Net ID: Identifies a network. The first several bits of a net ID, known as the class field or class bits, identify the class of the IP address.
Host ID: Identifies a host on a network.
IP addresses are divided into five classes, as shown in Figure 4. The shaded areas represent the address class. The first three classes are widely used.
Figure 4: IP address classes
Table 1 describes the address ranges of these five 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 | –– |
C | 192.0.0.0 to 223.255.255.255 | –– |
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. |