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 compose the host part of the IPv4 address, as the following figure illustrates.

Figure 5–4 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. The InterNIC assigns only the first byte of a class A number. Use of the remaining three bytes is left to the discretion of the owner of the network number. Only 127 class A networks can exist. Each one of these numbers can accommodate a maximum of 16,777,214 hosts.