IP Address and Classes

The Internet Protocol moves data between hosts in the form of datagrams. Each datagram is delivered to the address contained in the Destination Address (word 5) of the datagram's header. The Destination Address is a standard 32-bit IP address that contains sufficient information to uniquely identify a network and a specific host on that network.
An IP address contains a network part and a host part, but the format of these parts is not the same in every IP address. The number of address bits used to identify the network, and the number used to identify the host, vary according to the prefix length of the address. There are two ways the prefix length is determined: by address class or by a CIDR address mask.
Address Classes
Originally, the IP address space was divided into a few fixed-length structures called address classes. The three main address classes are class A, class B, and class C.
1. If the first bit of an IP address is 0, it is the address of a class A network. The first bit of a class A address identifies the address class. The next 7 bits identify the network, and the last 24 bits identify the host. There are fewer than 128 class A network numbers, but each class A network can be composed of millions of hosts.
2.If the first 2 bits of the address are 1 0, it is a class B network address. The first 2 bits identify class; the next 14 bits identify the network, and the last 16 bits identify the host. There are thousands of class B network numbers and each class B network can contain thousands of hosts.
3.If the first 3 bits of the address are 1 1 0, it is a class C network address. In a class C address, the first 3 bits are class identifiers; the next 21 bits are the network address, and the last 8 bits identify the host. There are millions of class C network numbers, but each class C network is composed of fewer than 254 hosts.
4. If the first 4 bits of the address are 1 1 1 0, it is a multicast address. These addresses are sometimes called class D addresses, but they don't really refer to specific networks. Multicast addresses are used to address groups of computers all at one time. Multicast addresses identify a group of computers that share a common application, such as a video conference, as opposed to a group of computers that share a common network.
5. If the first four bits of the address are 1 1 1 1, it is a special reserved address. These addresses are sometimes called class E addresses, but they don't really refer to specific networks. No numbers are currently assigned in this range.