System Administration Guide: IP Services

Class A Network Numbers

A class A network number uses the first 8 bits of the IPv4 address as its “network part.” The remaining 24 bits contain the host part of the IPv4 address, as the following figure illustrates.

Figure 10–3 Byte Assignment in a Class A Address

Diagram shows bits 0-7 is network part and remaining
24 bits are host part of a 32 bit IPv4 Class A address.

The values that are assigned to the first byte of class A network numbers fall within the range 0–127. Consider the IPv4 address The value 75 in the first byte indicates that the host is on a class A network. The remaining bytes, 4.10.4, establish the host address. Only the first byte of a class A number is registered with the IANA. Use of the remaining three bytes is left to the discretion of the owner of the network number. Only 127 class A networks exist. Each one of these numbers can accommodate a maximum of 16,777,214 hosts.