Glossary · Networking

IP Address Classes (Classful Addressing)

IP address classes were the original IPv4 scheme dividing addresses into classes A to E by leading bits, later replaced by CIDR.

Updated October 6, 2026

IP address classes are the original (classful) way of dividing the IPv4 address space, defined in the early 1980s. The value of the first octet decided how many bits belonged to the network portion and how many to the host, with Classes A, B and C used for ordinary unicast addresses.

Why it matters

  • Class A: first octet 1 to 126, default mask /8, about 16.7 million hosts per network.
  • Class B: 128 to 191, default mask /16, about 65,000 hosts.
  • Class C: 192 to 223, default mask /24, 254 hosts.
  • Class D: 224 to 239, used for multicast. Class E: 240 to 255, reserved.

The fixed sizes wasted addresses, since a company needing 1,000 hosts had to take a Class B with 65,000. Since 1993 classless addressing (CIDR) lets networks be any prefix length, such as /22. Classes remain useful for reading older documentation and exam material.

Example

192.168.10.5 begins with 192, so it is Class C and the classful mask is 255.255.255.0. The address 172.20.4.9 is Class B. Today both are shown with the prefix, such as 192.168.10.5/24, and the class has no effect on routing.

Common mistakes

  • Assuming a Class C address must use /24 on modern networks.
  • Forgetting that 127.0.0.0/8 is reserved for loopback.
  • Confusing classes with private ranges.

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