classful

Classful addressing denotes an early scheme for IPv4 address allocation in which networks were categorized into classes based on the value of the first octet. Under this scheme, the size of a network's address space—and therefore its fixed subnet mask—was determined by the class: Class A networks used a default mask of 255.0.0.0 (/8), Class B used 255.255.0.0 (/16), and Class C used 255.255.255.0 (/24). The assignment of a class effectively defined both the network portion and the host portion of an address, with the boundary between them set by rigid class rules.

History and usage: During the 1980s and early 1990s, Internet routing and address allocation relied on classful addressing. Addresses were allocated in blocks that matched those default masks, and routers often performed simpler, classful routing with fixed masks. Private address ranges were defined later by RFC 1918, providing non-routable space for internal networks.

Limitations: The rigid class boundaries led to address waste when organizations required more or fewer hosts than a class size would efficiently provide. Classful routing also hindered subnetting, because changing the subnet size often violated the class boundary and disrupted routing. This contributed to IPv4 address exhaustion and inflated routing tables.

Transition and legacy: The introduction of CIDR in the early 1990s, along with route aggregation and variable-length subnet masking (VLSM), replaced classful addressing for Internet routing. CIDR allows arbitrary network prefixes and more efficient use of addresses while reducing routing table size. Classful addressing remains a historical concept and is encountered in older configurations and some legacy devices.