In the early stages of Internet Protocol development, the network number was always the highest order octet (the eight most significant bits). Since this method was only allowed for 256 networks, it soon proved inadequate when additional networks were developed that were independent of the existing networks designated by the network number. In 1981, the addressing specification was revised with the introduction of the classful network architecture.
The classy network design allows for the assignment of larger individual networks and fine-grained subnetwork designs. The first three bits of the most significant octet of an IP address are defined as the address class. Three classes ( A , B, and C) are defined for universal unicast addressing. Depending on the inherited class, network identification is based on the octet boundary segment of the entire address. Each class uses successive additional octets in the network identifier, thereby reducing the number of hosts in the higher-order classes (B and C). The following table provides an overview of this now obsolete system.
Classful's network design served its purpose in the early stages of the Internet, but lacked the scalability in the face of the rapid network expansion of the 1990s. The system of class address space was replaced with Classless Inter-Domain Routing (CIDR) in 1993. CIDR is based on variable-length subnet masking (VLSM) to allow allocation and routing based on arbitrary-length prefixes. Today, remnants of the classful networking concept function only to a limited extent as a default configuration parameter of some network software and hardware components (eg netmasks), and in the technical jargon used in network administrator discussions.