These 32-bit addresses are the original IP addressing format that was designed for TCP/IP. Originally, IP networks have three classes, A, B, and C. The network number that is assigned to a network reflects this class designation plus 8 or more bits to represent a host. Class-based IPv4 addresses require you to configure a netmask for the network number. Furthermore, to make more addresses available for systems on the local network, these addresses were often divided into subnets.
Today, IP addresses are referred to as IPv4 addresses. Although you can no longer obtain class-based IPv4 network numbers from an ISP, many existing networks still have them. For more information about administering IPv4 addresses, refer to Designing Your IPv4 Addressing Scheme.