1. Classful addressing, formally adopted as part of the Internet Protocol (IP) in RFC 791, was the Internet's first major addressing scheme. The IP address was 32 bits in size, just as today, but was managed considerably differently.

   1. There were three address classes to chose from: A, B, or C, corresponding to 8-bit, 16-bit, or 24-bit prefixes. No other prefix lengths were allowed, and there was no concept of nesting a group of 24-bit prefixes, for example, within a 16-bit prefix.

   2. An address was slotted into one of three address classes based on its high-order bits. Addresses beginning with 0 were considered class A; addresses beginning 10 were class B; addresses beginning 110 class C. Two other classes were also defined, class D addresses beginning 1110 and class E addresses beginning 1111, though neither of these two address classes were normally used. For humans, the easiest way to distinguish between different address classes is to use the first decimal number in the IP address:

First octet ----- Address Class

0-127 ----- Class A

128-191----- Class B

192-223 -----Class C

224-239 -----Class D

240-255----- Class E