IP addresses and subnets

For its first years the internet handed out addresses in fixed sizes. A class C network held 256 addresses, a class B held 65,536, a class A about 16 million. An organisation that needed 2,000 addresses was too big for a C, so it got a B and left most of it unused.

By the early 1990s the class B space was running out and the routing tables of the backbone were growing faster than routers could cope. In September 1993 the IETF published RFC 1519 and threw the classes away. From then on, a network could be any size you could write after a slash.