Internet addresses are allocated by the InterNIC (https://www.internic.net ), the organization that administers the Internet. These IP addresses are divided into classes. The most common of these are classes A, B, and C. Classes D and E exist, but are not generally used by end users. Each of the address classes has a different default subnet mask. You can identify the class of an IP address by looking at its first octet. Following are the ranges of Class A, B, and C Internet addresses, each with an example address:

• Class A networks use a default subnet mask of 255.0.0.0 and have 0-127 as their first octet. The address 10.52.36.11 is a class A address. Its first octet is 10, which is between 1 and 126, inclusive.
• Class B networks use a default subnet mask of 255.255.0.0 and have 128-191 as their first octet. The address 172.16.52.63 is a class B address. Its first octet is 172, which is between 128 and 191, inclusive.
• Class C networks use a default subnet mask of 255.255.255.0 and have 192-223 as their first octet. The address 192.168.123.132 is a class C address. Its first octet is 192, which is between 192 and 223, inclusive.

In some scenarios, the default subnet mask values do not fit the needs of the organization, because of the physical topology of the network, or because the numbers of networks (or hosts) do not fit within the default subnet mask restrictions. The next section explains how networks can be divided using subnet masks.

 

Class A Network (/ 8 Prefixes)

This network is 8-bit network prefix. Its highest bit is set to 0, and contains a 7-bit network number and a 24-bit host number.

A maximum of 126, which is (2 ⁷ -2,) networks can be defined; two is subtracted because all an (0 and 1) subnet cannot be used in certain routers using RIP-1 Protocol. Each network supports a maximum of 16,777,214 (2 ²⁴ -2) hosts per network. You must subtract two because the base network represents host “0”, and the last host on the network is actually used for 1s ("broadcast") and may not be assigned to any host.

The class A network address block contains 2 ³¹ power (2,147,483,648) individual addresses. The IPv4 address space contains a maximum of 2 ³² power (4,294,967,296) addresses, which mean that a class A network address space is 50% of the total IPv4 unicast, address space.

Class B Networks (/16 Prefixes)

This network is a 16-bit network prefix; its highest bit order is set to 1-0. It is a 14-bit network number with a 16-bit host number.

This class defines 16,384 (2 ¹⁴ ) /16 networks, and supports a maximum of 65,534 (2 ¹⁶ -2) hosts per network. Class B /16 block address is (1,073,741,824) = 2 ³⁰; therefore it represent 25% of the total IPV4.

Class C Networks (/24 Prefixes)

This is a 24-bit network prefix; it has a 3 bit set to the highest order 1-1-0. It is a 21-bit network number with 8-bit host number.

This class defines a maximum of 2,097,152 (2 ²¹ ) /24 networks. And each network supports up to 254 (2 ⁸ -2) hosts. The entire class C network represents 2 ²⁹ (536,870,912) addresses; therefore it is only 12.5 % of the total IPv4.

Other Networks

There are two other networks that are not commonly used, class D and Class E.
Class D has its highest bit order set to 1-1-1-0 it is used to support multicasting.
Class E has its highest bit order set to 1-1-1-1 which is reserved for experimental use.