IP addressing, when started a few decades ago, used the concept of classes. This architecture is called class full IP addressing. In the mid of 1990s, a new architecture, Called classless IP addressing was introduced that will eventually supersede the original architecture. However, most of the internet is still using calssfull addressing and the migration is slow. To understand the classfull IP addressing it is important to understand classless addressing.

In classfull IP addressing, the IP address is divided into five classes A,B,C,D and E. each class is occupies some part of the whole address space, see below the table mansion the class occupation of the address space (approximate).

We can see that the class A covers the half of the address space, a serious design flow. Class B cover ¼ of the whole address space, another design flow. Class C is cover the 1/8 of the address, and class D and E each cover the 1/16 of the address space.

We can find the class of an address when the address is given in binary notation or in dotted decimal notation.

Find the class in binary notation

If the address is given in binary notation, the first bits can immediately tell us the class of the address.

Find the class in dotted decimal notation

When the address is given in dotted decimal notation, then we need to look only at the first byte to determining the class of the address. Each class has a specific range of number.

Net ID and Host ID

In classfull addressing an IP in classes A,B and C is divided into net id and host id.

Note: Class D and E are not divided into net id and host id.