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.
The values that are assigned to the first byte of class A network numbers fall within the range 0–127. Consider the IPv4 address 75.4.10.4. 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.