Internet Core Protocols: The Definitive Guide

Subnet Classes

Until recently, network addresses were clumped into three distinct classes, each of which provided different-sized blocks of network addresses for organizations to use on their internal networks. The only real difference between these network classes was the number of bits used in the subnet mask to identify the network portion of the addresses, which in turn dictated the number of hosts that could be used on that network. These address classes are shown in Table 2.2.

Table B.2. Common Subnet Mask Classes

Class	Network Portion	Host Portion
A	11111111	00000000 00000000 00000000
B	11111111 11111111	00000000 00000000
C	11111111 11111111 11111111	00000000

The number of networks available with each of the subnet classes—and the number of hosts possible on each of those networks—varies widely between the different classes. This concept is illustrated in Table 2.3, which shows that there are only a few Class A networks available, although each of them can have millions of possible hosts. Conversely, there are a couple of million possible Class C networks, although they can only serve 254 devices each (after subtracting the all-ones and all-zeroes addresses).

Table B.3. Available Number of Networks and Hosts for the Default Subnet Classes

Class	Network Bits	Nets per Class	Host Bits	Hosts per Net
A	7	125	24	16,777,213
B	14	16,383	16	65,533
C	21	2,097,151	8	254

Networks that are very large would want to use the Class A subnet mask, since Class A networks can have millions of nodes on a single large network. ...