IPV4 Addressing | Classful and Classless Addressing

In this tutorial, we will learn about the IPV4 Addressing - Classful and Classless Addressing in Computer Network. By Radib Kar Last updated : May 04, 2023

1) Classful Addressing

IPv4 addressing used the concept of classes. This architecture is known as classful addressing.

In the classful addressing, there are 5 classes in which the address space is divided: A, B, C, D, and E.

Each class occupies some fraction of the address space.

We can find the class of an address when given the address in binary notation or dotted-decimal notation by checking the first few bits or first byte.

Classes and Blocks

There is a problem with the classful addressing that is "each class is divided into a fixed number of blocks with each block having a fixed size".

Class name Number of blocks Block size Application
A 128 16,777,216 Unicast
B 16,384 65,536 Unicast
C 2,097,152 256 Unicast
D 1 268,435,456 Multicast
E 1 268,435.456 Multicast

The "class A addresses" are designed for large organizations to manage a large number of attached hosts or routers.

The "class B addresses" are designed for midsize organizations to manage tens of thousands of attached hosts or routers.

The "class C addresses" are designed for small organizations to manage a small number of attached hosts or routers.

2) Classless Addressing

Classful addressing leads to address depletion. That's the big issue for this schema and that's why it's not used nowadays.

To overcome the problem of address depletion and to give more organizations access to the Internet, the classless addressing was designed and implemented. In this scheme of classless addressing, there are no classes, but the addresses are still granted in blocks.

Address Blocks

In classless addressing, when an entity(organization or a single household (small organization) or whatever which uses the internet) needs to be connected to the Internet, it is granted a block (range) of addresses. The size of the block (the number of addresses) varies based on the nature, size, and need of the entity.

For example, a household (small organization) may be given only two addresses; a large organization may be given thousands of addresses. On the other hand. An ISP, as the Internet service provider, may be given hundreds of thousands based on the number of customers it may serve.

Three restrictions on classless address blocks:

  1. The addresses in a block must be contiguous that means one after another.
  2. The number of classless addresses in a block must be a power of 2.
  3. The first address must be evenly divisible by the number of addresses.


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