Problem with IP Address Classes

  • companies would receive class A (16,777,214 hosts), B (65,534 hosts) or C (254 hosts) classes depending on the number of hosts needed
  • jumps between allocatable hosts within these classes are huge
    • thus, huge amount of global address space was wasted
  • solution: Classless Inter-Domain Routing (CIDR)

CIDR as partial solution

  • CIDR: removed /8, 16/ and /24 requirements for the address classes
    • allowing for them to be split or subnetted into smaller networks
      • e.g. 175.10.10.0/20
    • thus, address ranges can be allocated which more closely match existing needs reducing the waste of address space
  • Route Summarisation: aggregate blocks of networks can be advertised on the internet
    • let’s say
      • ISP A (internet service provider) gives out 256 address blocks (175.10.0.0/24, 175.10.1.0/24, 175.10.2.0/24, …, 175.10.255.0/24)
      • ISP B gives out another 256 address blocks (175.11.0.0/24, 175.11.1.0/24, 175.11.2.0/24, …, 175.11.255.0/24)
      • ISP A and ISP B get connected
      • without CIDR, ISP A would advertise all 256 address blocks to ISP B and vice versa
    • route summarisation allows to advertize an aggregate block
      • ISP A can advertise 175.10.0.0/16 to ISP B
      • ISP B can advertise 175.11.0.0/16 to ISP A
    • each ISP only needs to know the one summary route rather than 256 individual routes
      • reduces each ISP’s routing table and takes up less memory
        • no need to recalculate routing tables if a link goes down in the other ISP’s network