Class A addresses

A Class A address consists of an 8-bit network address and a 24-bit local or host address.

The first bit in the network address is dedicated to indicating the network class, leaving 7 bits for the actual network address. Because the highest number that 7 bits can represent in binary is 128, there are 128 possible Class A network addresses. Of the 128 possible network addresses, two are reserved for special cases: the network address 127 is reserved for local loopback addresses, and a network address of all ones indicates a broadcast address.

There are 126 possible Class A network addresses and 16,777,216 possible local host addresses. In a Class A address, the highest order bit is set to 0.

This illustration shows a typical class A address structure. The first 8 bits contain the network address (always beginning with a zero). The remaining 24 bits contain the local host address.

The first octet of a Class A address is in the range 1 to 126.