What are the classifications of classful IP addresses?

IP addresses are generally represented by a 32-bit unsigned binary value in dotted decimal format. For example, 192.168.1.1 is a valid IP address consisting of four octets, each containing 8 bits.

The IP address consists of two main components:

IP address = <network number><host number>

The first bits of the IP address specify how the rest of the address should be separated into its network and host portions. This addressing scheme is known as classful addressing, which divides the IP address space into distinct classes based on the leading bits.

Classes of IP Addresses

There are five classes of IP addresses in classful addressing:

• Class A − Starts with binary '0'
• Class B − Starts with binary '10'
• Class C − Starts with binary '110'
• Class D − Starts with binary '1110' (Multicast)
• Class E − Starts with binary '1111' (Reserved)

Class A Networks

Class A addresses use 8 bits for the network portion and 24 bits for the host portion. The first bit is always 0, leaving 7 bits for the actual network identifier.

• Range: 1.0.0.0 to 126.255.255.255 (0 and 127 are reserved)
• Networks: 126 possible networks (2⁷ - 2)
• Hosts per network: 16,777,214 (2²⁴ - 2)
• Default subnet mask: 255.0.0.0 (/8)

Class B Networks

Class B addresses use 16 bits for the network portion and 16 bits for the host portion. The first two bits are always '10'.

• Range: 128.0.0.0 to 191.255.255.255
• Networks: 16,384 possible networks (2¹⁴)
• Hosts per network: 65,534 (2¹⁶ - 2)
• Default subnet mask: 255.255.0.0 (/16)

Class C Networks

Class C addresses use 24 bits for the network portion and 8 bits for the host portion. The first three bits are always '110'.

• Range: 192.0.0.0 to 223.255.255.255
• Networks: 2,097,152 possible networks (2²¹)
• Hosts per network: 254 (2⁸ - 2)
• Default subnet mask: 255.255.255.0 (/24)

Class D and Class E

Class D addresses (224.0.0.0 to 239.255.255.255) are reserved for multicast communications. These addresses do not have separate network and host portions and are used for group communication and broadcasting to multiple recipients simultaneously.

Class E addresses (240.0.0.0 to 255.255.255.255) are reserved for experimental use and future applications. These addresses are not used in regular networking operations.

Comparison of IP Address Classes

Conclusion

Classful IP addressing divides the IPv4 address space into five distinct classes (A, B, C, D, E) based on the leading bits, with Classes A, B, and C used for unicast communication and different network sizes. While this system has been largely replaced by CIDR in modern networking, understanding classful addressing remains important for network fundamentals and legacy system compatibility.