When the IEEE committee sat down to sort out the range of numbers that were going to be used by all computers, they came out with 5 different ranges or, as we call them, "Classes" of IP Addresses and when someone applies for IP Addresses they are given a certain range within a specific "Class" depending on the size of their network.
To keep things as simple as possible, let's first have a look at the 5 different Classes:
In the above table, you can see the 5 Classes. Our first Class is A and our last is E. The first 3 classes ( A, B and C) are used to identify workstations, routers, switches and other devices whereas the last 2 Classes ( D and E) are reserved for special use.
As you would already know an IP Address consists of 32 Bits, which means it's 4 bytes long. The first octec (first 8 Bits or first byte) of an IP Address is enough for us to determine the Class to which it belongs. And, depending on the Class to which the IP Address belongs, we can determine which portion of the IP Address is the Network ID and which is the Node ID.
For example, if I told you that the first octec of an IP Address is "168" then, using the above table, you would notice that it falls within the 128-191 range, which makes it a Class B IP Address.
Understanding the Classes
We are now going to have a closer look at the 5 Classes. If you remember earlier I mentioned that companies are assigned different IP ranges within these classes, depending on the size of their network. For instance, if a company required 1000 IP Addresses it would probably be assigned a range that falls within a Class B network rather than a Class A or C.
The Class A IP Addresses were designed for large networks, Class B for medium size networks and Class C for smaller networks.
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