This article tells basic of IP Address and what was reason behind transition from classful addressing to classless addressing CIDR

Internet Protocol (IP) Address is a logical address provided to each network device connected in a computer network for routing info. Its basic purpose is to uniquely identify each node connected in the network.

Initially TCP/IP designers designed IP address (IPv4) as 32 bits number which is stored in Binary forms but for human readability these numbers are represented in dotted decimal format for example 210.23.45.89. IP address has two parts Network ID and Host ID. The first part tells about the network in which Host is connected and the second part identify Host in that particular network. Network ID is also known as the network prefix as it is first part of IP address. All devices connected in a particular network have same network prefix but different host ID.

To provide flexibility designers decided to divide IP addresses in three address classes Class A, Class B and Class C. This is also known as classful addressing. Each class differs in terms of bits used for network and host IDs.

Class A- Leading Bit 0, Network bits 8, Host bits 24

Class B- Leading Bits 10, Network bits 16, Host bits 16

Class C- Leading Bits 110, Network bits 24, Host bits 8

Total number of networks in particular class is 2n where n is no of bits for network ID.

Total number of hosts is 2n-2, where n is no of bits in host id. Two addresses all 0 host bits and all 1 host bits are used for network and broadcast addresses respectively.

Classful addressing does not provide efficient allocation of addresses. Class B can have 65,534 hosts while class C can have only 254 hosts. Any organization in need of few hundred hosts was assigned Class B addresses which resulted in depletion of Class B addresses.

In CIDR (Classless Inter Domain Routing) which is based on VLSM (Variable Length Subnet Masking) we can use arbitrary length network prefix.