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.
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).
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. ...
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