IPv4 Classful Addressing

To provide flexibility in the number of addresses distributed to networks of different sizes, 4-octet (32-bit) IP addresses were originally divided into three different categories or classes: class A, class B, and class C. Each address class specifies a different number of bits for its network prefix and host number:

• Class A addresses use only the first byte (octet) to specify the network prefix, leaving 3 bytes to define individual host numbers.
• Class B addresses use the first 2 bytes to specify the network prefix, leaving 2 bytes to define host addresses.
• Class C addresses use the first 3 bytes to specify the network prefix, leaving only the last byte to identify hosts.

In binary format, with an x representing each bit in the host number, the three address classes can be represented as follows:

00000000 xxxxxxxx xxxxxxxx xxxxxxxx (Class A)
00000000 00000000 xxxxxxxx xxxxxxxx (Class B)
00000000 00000000 00000000 xxxxxxxx (Class C)

Because each bit (x) in a host number can have a 0 or 1 value, each represents a power of 2. For example, if only 3 bits are available for specifying the host number, only the following host numbers are possible:

111 110 101 100 011 010 001 000

In each IP address class, the number of host-number bits raised to the power of 2 indicates how many host numbers can be created for a particular network prefix. Class A addresses have 2²⁴ (or 16,777,216) possible host numbers, class B addresses have 2¹⁶ (or 65,536) host numbers, and class C addresses have 2⁸ (or 256) possible host numbers.

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