Configure TCP/IP - MCSA Windows Server 2012 R2: Installation and Configuration Study Guide - page 395

In the following sections, you will look at the three network types.
Class A Networks
In a Class A network, the first byte is the network address, and the three remaining bytes
are used for the node addresses. The Class A format is Network.Node.Node.Node.
For example, in the IP address 49.22.102.70, 49 is the network address, and 22.102.70
is the node address. Every machine on this particular network would have the distinctive
network address of 49. Within that network, however, you could have a large number of
machines.
There are 126 possible Class A network addresses. Why? The length of a Class A
network address is 1 byte, and the first bit of that byte is reserved, so 7 bits in the first
byte remain available for manipulation. This means that the maximum number of Class A
networks is 128. (Each of the 7 bit positions that can be manipulated can be either a 0 or a
1, and this gives you a total of 2 7 positions, or 128.) But to complicate things further, it was
also decided that the network address of all 0s (0000 0000) would be reserved. This means
that the actual number of usable Class A network addresses is 128 minus 1, or 127. Also,
127 is a reserved number (a network address of 0 followed by all 1s [0111 1111], so you
actually start with 128 addresses minus the 2 reserved, and you're left with 126 possible
Class A network addresses.
Each Class A network has 3 bytes (24 bit positions) for the node address of a
machine, which means that there are 2 24 , or 16,777,216, unique combinations. Because
addresses with the two patterns of all 0s and all 1s in the node bits are reserved, the actual
maximum usable number of nodes for a Class A network is 2 24 minus 2, which equals
16,777,214.
Class B Networks
In a Class B network, the first 2 bytes are assigned to the network address, and the
remaining 2 bytes are used for node addresses. The format is Network.Network.Node.
Node.
For example, in the IP address 130.57.30.56, the network address is 130.57, and the
node address is 30.56.
The network address is 2 bytes, so there would be 2 16 unique combinations. But the
Internet designers decided that all Class B networks should start with the binary digits 10.
This leaves 14 bit positions to manipulate; therefore, there are 16,384 (or 2 14 ) unique Class
B networks.
This gives you an easy way to recognize Class B addresses. If the first 2 bits of the first
byte can be only 10, that gives you a decimal range from 128 up to 191 in the first octet
of the IP address. Remember that you can always easily recognize a Class B network by
looking at its first byte, even though there are 16,384 different Class B networks. If the first
octet in the address falls between 128 and 191, it is a Class B network, regardless of the
value of the second octet.