http://en.wikipedia.org/wiki/Classful_network
Classful network is a term that is used to describe the network architecture of the
Internet until around 1993. It divided the
address space for Internet Protocol Version 4 (
IPv4) into five address classes. Each class, coded by the first three bits of the address, defined a different size or type (unicast or multicast) of the network.
Today, remnants of classful network concepts remain in practice only in a limited scope in the default configuration parameters of some network software and hardware components (e.g. netmask), but the terms are often still heard in general discussions about network structure among network administrators.
The replacement of classes
This first round of changes was enough to work in the short run, but an
IP address shortage still developed. The principal problem was that most sites were too big for a "class C" network number, and received a "class B" number instead. With the rapid growth of the Internet, the available pool of class B addresses (basically 214, or about 16,000 total) was rapidly being depleted. Classful networking was replaced by
Classless Inter-Domain Routing (CIDR), starting in about 1993, to solve this problem (and others).
Early allocations of IP addresses by
IANA were in some cases not made very efficiently, which contributed to the problem. (However, the commonly held notion that some American organizations unfairly or unnecessarily received class A networks is a
canard; most such allocations date to the period
before the introduction of address classes, when the only thing available was what later became known as "class A" network number.)
