A class A address could be diagramed:
Network Host
+------+ +----------------------+
| | | |
[0xxxxxxx][xxxxxxxxxxxxxxxxxxxx
xxxx]
which shows the eight network bits followed by the 24 host bits.
These 32-bit IP addresses are almost always written as four dot-separated
decimal numbers, one for each byte of the address. Thus, our class A address
would have a range of address numbers from 1.0.0.0 through
126.0.0.0 ( 0. x.x.x and 127.
x.x.x are reserved). The number of host addresses per network is
16,777,214, which is two less than two raised to the 24th power because both
host numbers 0.0.0 and 255.255.255 are reserved.
In practice, people don't really attach 16 million hosts to a network so
administrators of a Class A site often divide the host address portion into a (sub)network
and host portion. (Subnetting is now supported by most operating systems.) Each
Class A network number can support up to 65,534 subnets (network numbers
0.0 and 255.255 are reserved) with each having 254 hos ts
(host numbers 0 and 255 are reserved). This is done by
using the 16 high -order bits of the host portion for the subnet number and the
lower eight bits for the host as diagramed here:
Network Subnet Host
+------+ +--------------+ +------+
| | | | | |
[0xxxxxxx][xxxxxxxxxxxxxxxx][xxxxxxxx]
Class B Addresses
The first two bits of a Class B address are 1 and 0, the next fourteen bits
identify the network and the last sixteen the host, as diagramed:
Network Host
+--------------+ +--------------+
| | | |
[10xxxxxxxxxxxxxx][xxxxxxxxxxxxxxxx]
Thus, Class B addresses include the network numbers in the range from
128.1.0.0 through 191.254.0.0 for a total of 65,534 host
addresses.
As with the Class A address, we can divide the host portion of a Class B
address into subnet and host parts. For instance, let's spli t our Class B
network number on the byte boundary, that is, the eight MSBs of the host portion
identifies the subnet and the remaining bits the host, as diagramed:
Network Subnet Host
+--------------+ +------+ +------+
| | | | | |
[10xxxxxxxxxxxxxx][xxxxxxxx][xxxxxxxx]
This arrangement allows 254 subnets each with 254 hosts.
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