What is classful IP addressing?

Classful IP addressing is an IPv432-bit IP addressing scheme addressing architecture that is divided into five classes as follows:

• Class A
• Class B
• Class C
• Class D
• Class E

Class A

In class A, the first 8 bits are for the network part of the address, and the remaining 24 bits are reserved for the host part of the address. The leading first bit of the octet is fixed.

We subtract two addresses from the network addresses since they are considered special addresses.

Usable addresses are as follows:

• Network addresses $2^7-2= 126$
• Host addresses $2^{24}-2 = 16,777,214$

Class A is used for large-sized networks.

Note: The IP range 127.x.x.x is reserved for loopbackPackets sent through this IP never reach the destination but are looped back to the network card only. It is used for diagnostics purposes..

Class B

In class B, the first 16 bits are for the network part of the address, and the remaining 16 bits are reserved for the host part of the address. The leading first two bits of the octet are fixed.

Usable addresses are as follows:

• Network addresses $2^{14} = 16384$
• Host addresses $2^{16} – 2 = 65534$

Class B is used for medium-sized networks.

Class C

In class C, the first 24 bits are for the network part of the address, and the remaining 8 bits are reserved for the host part of the address. The leading first three bits of the octet are fixed.

Usable addresses are as follows:

• Network addresses $2^{21} = 2097152$
• Host addresses $2^8 – 2 = 254$

Class C is used for small-sized networks.

Class D

Class D is reserved for multicasting. Higher-order bits of the octet are set as $1110$.

Class E

Class E is reserved for experiment and research purposes. Higher-order bits of the octet are set as$1111$.

Summary

Limitation

Internet uses class A, B, and C addresses. The issue with IPv4 classful addressing is that thousands of class A addresses are wasted, including many class B addresses and a few class C addresses that are insufficient to meet the demands of companies. 

Due to these issues, Classless Inter-Domain Routing (CIDR) has taken the place of classful networking.