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
- 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