The classful addressing specified in RFCs 790 and 791 resulted in a tremendous waste of address space. In the early days of the Internet, organizations were assigned an entire classful network address from the A, B, or C class.
As illustrated in the figure:
- Class A had 50% of the total address space. However, only 126 organizations could be assigned a class A network address. Ridiculously, each of these organizations could provide addresses for up to 16 million hosts. Very large organizations were allocated entire class A address blocks. Some companies and governmental organizations still have class A addresses. For example, General Electric owns 3.0.0.0/8, Apple Computer owns 17.0.0.0/8, and the U.S. Postal Service owns 56.0.0.0/8.
- Class B had 25% of the total address space. Up to 16,384 organizations could be assigned a class B network address and each of these networks could support up to 65,534 hosts. Only the largest organizations and governments could ever hope to use all 65,000 addresses. Like class A networks, many IP addresses in the class B address space were wasted.
- Class C had 12.5 % of the total address space. Many more organizations were able to get class C networks, but were limited in the total number of hosts that they could connect. In fact, in many cases, class C addresses were often too small for most midsize organizations.
- Classes D and E are used for multicasting and reserved addresses.
The overall result was that the classful addressing was a very wasteful addressing scheme. A better network addressing solution had to be developed. For this reason, Classless Inter-Domain Routing (CIDR) was introduced in 1993.