IP address classes

IP addresses are divided into five classes that are identified by the value of the first octet (the first decimal number). The system of IP address classes was developed for the purpose of Internet IP addresses assignment. The classes created were based on the network size. For example, for the small number of networks with a very large number of hosts, the Class A was created. The Class C was created for the numerous networks with the small number of hosts.

The IP address classes are:

• Class A, 0-127 – for example, 10.50.13.40. For large networks with many devices.
• Class B, 128-191 – for example, 130.5.4.77. For medium-sized networks.
• Class C, 192-223 – for example, 192.168.5.10. For small networks with the small number of hosts.
• Class D, 224-239 – for example, 224.0.0.5. For multicast addresses.
• Class E, 240-255 – for example, 241.0.0.1. Experimental.

Reserved addresses (used for special purposes):

• 0.0.0.0/8 – used to communicate with the network the device is on.
• 127.0.0.0/8 – loopback addresses.
• 169.254.0.0/16 – link-local addresses (APIPA).

An IP address consists of 32 bits. These bits are divided into two parts:

• network bits – identify a particular network.
• host bits – identify a host on the network.

For the IP addresses from Class A, the first 8 bits (the first decimal number) represent the network part, while the remaining 24 bits represent the host part. For Class B, the first 16 bits (the first two numbers) represent the network part, while the remaining 16 bits represent the host part. For Class C, the first 24 bits represent the network part, while the remaining 8 bits represent the host part. For example, consider the following IP addresses:

10.50.120.7 – because this is a Class A address, the first number (10) represents the network part, while the remainder of the address represents the host part (50.120.7). This means that, in order for devices to be on the same network, the first number of their IP addresses has to be the same for both devices. In this case, a device with the IP address of 10.47.8.4 is on the same network as the device with the IP address listed above. The device with the IP address 11.5.4.3 is not on the same network, because the first number of its IP address is different.

172.16.55.13 – because this is a Class B address, the first two numbers (172.16) represents the network part, while the remainder of the address represents the host part (55.13). The device with the IP address of 172.16.254.3 is on the same network, while a device with the IP address of 172.55.54.74 isn’t.