Three Sizes Fit All
How many IP addresses can you have in network 1.0.0.0? As it
turns out, you can't use the number 1.0.0.0 because that's the network number.
You also can't use 1.255.255.255; that number is reserved for other reasons.
However, any other IP address that starts with 1 would be valid. If you counted
all the addresses, you would end up with more than 16 million addresses!
IP defines these three sizes of networks as different classes of networks. The three different network
classes are called Class A, B, and C. By definition, all addresses in the same
Class A, B, or C network have the same numeric value network portion of the addresses. The rest of the
address is called the host portion of the
address. For example, in a Class A network such as network 1.0.0.0, all
addresses begin with the number 1 in the first octet. The last 3 octets of these
addresses comprise the host part of the address.
When the TCP/IP RFCs first defined the concept of networks,
such as networks 1.0.0.0, 2.0.0.0, and 3.0.0.0, those same RFCs defined two
other classes of networks. Whereas networks 1.0.0.0, 2.0.0.0, and 3.0.0.0
provide more than 16 million addresses each, another (class B) of network gives
us a little more than 65,000 addresses. Still another (class C) gives us 254
addresses per IP network. The people who designed TCP/IP addressing simply
decided that the world would need three sizes of networks.
Comparing Class A, B, and C addresses, they each have a
different length for the network and host part of the addresses. By doing so,
each class of network allows a different number of host addresses. To understand
that fully, first consider the formats of the three classes of IP networks, as
listed in Table 10-1.
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