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 Classless Interdomain Routing (CIDR) address mask. We begin with a discussion of traditional IP 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. By examining the first few bits of an address, IP software can quickly determine the class, and therefore the structure, of an address. IP follows these rules to determine the address class:
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 ...