Classful Addressing

Overview

Classful addressing is the original method of categorizing IPv4 addresses into five distinct classes (A through E). This addressing scheme, introduced in 1981, provided a structured way to allocate IP addresses based on organization size. Although largely superseded by Classless Inter-Domain Routing (CIDR), understanding classful addressing remains important for legacy systems and networking fundamentals.

Detailed Explanation

Address Classes

  1. Class A Networks

    • First bit: 0
    • Range: 1.0.0.0 to 126.0.0.0
    • Default mask: 255.0.0.0 (/8)
    • Large organizations
    • 16,777,214 hosts per network

  2. Class B Networks

    • First bits: 10
    • Range: 128.0.0.0 to 191.255.0.0
    • Default mask: 255.255.0.0 (/16)
    • Medium-sized organizations
    • 65,534 hosts per network

  3. Class C Networks

    • First bits: 110
    • Range: 192.0.0.0 to 223.255.255.0
    • Default mask: 255.255.255.0 (/24)
    • Small organizations
    • 254 hosts per network

  4. Class D Networks

    • First bits: 1110
    • Range: 224.0.0.0 to 239.255.255.255
    • Multicast addresses
    • No host portion

  5. Class E Networks

    • First bits: 1111
    • Range: 240.0.0.0 to 255.255.255.255
    • Reserved for experimental use
    • Not used in public internet

Characteristics and Limitations

  1. Fixed Boundaries

    • Predefined network/host portions
    • Inflexible allocation
    • Inefficient address utilization
    • Limited subnet capability

  2. Historical Significance

    • Original Internet addressing scheme
    • Foundation for modern addressing
    • Legacy system compatibility
    • Educational importance

References

  • RFC 791 - Internet Protocol
  • RFC 1519 - Classless Inter-Domain Routing (CIDR)
  • Comer, D.E. (2000). Internetworking with TCP/IP