When the IP address scheme was developed, only
the first octet was used to identify the network
portion of the address. At the time it was assumed
that 254 networks would be more than enough to
cover the research groups and universities using
this protocol. As usage grew, however, it became
clear that more network designations would be
needed (each with fewer hosts). This issue led to
the development of address classes.
Addresses are segmented into five classes (A
through E). Classes A, B, and C are the most common.
Class A has 8 network bits and 24 host bits.
Class B has 16 network bits and 16 host bits, and
Class C has 24 network bits and 8 host bits. This
scheme was based on the assumption that there
would be many more small networks (each with
fewer endpoints) than large networks in the world.
Class D is used for multicast, and Class E is
reserved for research. The following table breaks
down the three main classes. Note that the Class A
address starting with 127 is reserved.