Classful IP addressing is a system used to divide the 32-bit IPv4 address space into different address “classes” for easier allocation of IP addresses.

There are five main classes: A, B, C, D, and E, but the classes that are primarily used for host addressing are A, B, and C.

Class A (Network Class A)

• Address Range: 0.0.0.0 to 127.255.255.255
• First Octet Range: 0 to 127 (binary: 00000000 to 01111111)
• Default Subnet Mask: 255.0.0.0 (or /8)
• Number of Networks: 128 (2^7)
• Number of Hosts per Network: 16,777,214 (2^24 – 2, since two addresses are reserved for network and broadcast)
• Usage: Typically used by large organizations or the internet backbone, as it can accommodate a vast number of hosts on a single network.
• Example: 10.0.0.1

Class A Structure

• Network Portion: 8 bits (First Octet)
• Host Portion: 24 bits (Remaining 3 Octets)

Class B (Network Class B)

• Address Range: 128.0.0.0 to 191.255.255.255
• First Octet Range: 128 to 191 (binary: 10000000 to 10111111)
• Default Subnet Mask: 255.255.0.0 (or /16)
• Number of Networks: 16,384 (2^14)
• Number of Hosts per Network: 65,534 (2^16 – 2)
• Usage: Used by medium-sized networks like universities or large corporations.
• Example: 172.16.0.1

Class B Structure

• Network Portion: 16 bits (First 2 Octets)
• Host Portion: 16 bits (Remaining 2 Octets)

Class C (Network Class C)

• Address Range: 192.0.0.0 to 223.255.255.255
• First Octet Range: 192 to 223 (binary: 11000000 to 11011111)
• Default Subnet Mask: 255.255.255.0 (or /24)
• Number of Networks: 2,097,152 (2^21)
• Number of Hosts per Network: 254 (2^8 – 2)
• Usage: Commonly used for smaller networks, such as small businesses or home networks.
• Example: 192.168.1.1

Class C Structure

• Network Portion: 24 bits (First 3 Octets)
• Host Portion: 8 bits (Last Octet)

Classful Addressing Summary Table