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IP Address Classes Overview

Sep 2, 2025

Overview

This lecture explains the concept of IP address classes in IPv4, their historical relevance, and the key subnet calculations for network design.

IP Address Classes

  • IPv4 addresses were historically divided into classes: A, B, C, D, and E.
  • Class A: first octet 0-127, default subnet mask 255.0.0.0, 8 bits for network and 24 for host.
  • Class B: first octet 128-191, default subnet mask 255.255.0.0, 16 bits for network and 16 for host.
  • Class C: first octet 192-223, default mask 255.255.255.0, 24 bits for network and 8 for host.
  • Class D: first octet 224-239, used for multicast, not assigned to hosts.
  • Class E: first octet 240-255, reserved for experimental use.
  • Class-based addressing is no longer used for subnetting since 1993, but terms are still used as a reference.

Identifying IP Classes

  • Check the first octet of the IP address to determine its class.
    • 17.x.x.x is Class A.
    • 220.x.x.x is Class C.
    • 165.x.x.x and 128.x.x.x are Class B.
    • 191.x.x.x is Class B.
    • 192.x.x.x is Class C.
  • In binary, Class A starts with 0, Class B with 10, Class C with 110, Class D with 1110, and Class E with 1111.

Key Subnet Calculations

  • Four important subnet values: network address, first host address, broadcast address, last host address.
  • Network address: set all host bits to zero.
  • First host address: one more than the network address.
  • Broadcast address: set all host bits to one.
  • Last host address: one less than the broadcast address.

Subnet Calculation Examples

  • For 10.74.22.11 (Class A, mask 255.0.0.0):
    • Network address: 10.0.0.0
    • First host: 10.0.0.1
    • Broadcast: 10.255.255.255
    • Last host: 10.255.255.254
  • For 172.16.188.200 (Class B, mask 255.255.0.0):
    • Network address: 172.16.0.0
    • First host: 172.16.0.1
    • Broadcast: 172.16.255.255
    • Last host: 172.16.255.254
  • For 192.168.4.91 (Class C, mask 255.255.255.0):
    • Network address: 192.168.4.0
    • First host: 192.168.4.1
    • Broadcast: 192.168.4.255
    • Last host: 192.168.4.254

Subnetting Process

  • The same subnetting formula applies regardless of class-based or modern subnetting approaches.

Key Terms & Definitions

  • IP address — A unique identifier for a device on a network.
  • Subnet mask — Determines which part of an IP address is the network and which is the host.
  • Network address — The address identifying the network itself, host bits set to zero.
  • Broadcast address — An address used to communicate with all devices on a subnet, host bits set to one.

Action Items / Next Steps

  • Review additional subnetting videos to reinforce the calculation process.
  • Practice identifying IP classes and calculating subnet addresses from given IPs.

Full transcript