In 1981, classful addressing, formally adopted as part of the Internet Protocol (IP) in RFC 790, was the Internet’s first major addressing scheme. The IP address was 32 bits in size, just like today. There were three address classes to choose from A, B, or C, corresponding to 8-bit, 16-bit, or 24-bit prefixes. No other prefix lengths allowed at that time, and there was no concept of nesting a group of 24-bit prefixes, such as, within a 16-bit prefix.
There are also two other classes: class D addresses and class E addresses, however, neither of these two address classes were normally used. Class D address is used for Multicasting and Class E is an experimental address and reserved for future use. For humans, the easiest way to distinguish between different address classes is to use the first decimal number in the IP address. The figure below all aspects of IP classes.
Classful networks use the classful subnet mask according to the leading bits in the first block of the IP address. Following is the detail of first three classes:-
Class A (0.0.0.0 to 127.255.255.255)
The default subnet mask for this class is 255.0.0.0 or /8. This class is support extremely large network with more than 16 million hosts. High order bit of Class A addresses should zero, so a zero creating total 128 possible class A network.
Class B (126.96.36.199 – 188.8.131.52)
The default subnet mask for this class is 255.255.0.0 or /16. This class supports the large networks up to 65,000 host addresses. The high-order bits for this class is 10 in the first octet and the remaining bits of the first 2 octets create over 16,000 networks.
Class C (192.0.0.0 – 184.108.40.206)
The default subnet mask for this class is 255.255.255.0 or /24. This class is support small networks with a maximum of 254 hosts. The first three bits of the octet indicate the high order bit. The remaining bits of the first octet; 2nd octet, and 3rd octets indicate the network and the 4th octet indicate host addresses in this class. The high-order bit is 110. Class C address has over 2 million possible networks.