Improvement on Classful Addressing
Uses variable length subnet masking - network prefixes have variable length
- See Subnets for info on subnet masks
Example: /8 means the network prefix size is 8 (class A)
IP Forwarding
Note
CIDR makes packet forwarding harder
Because prefixes are variable length, can’t use old forwarding method
Need to find longest match between an IP address and the variable length prefixes
Multi-homing: A host might use multiple providers
- One provider might be the primary and the other might be a backup/secondary
Problem with multi-homing
- Provider 1 has aggregated the four subnets
- The yellow address (201.10.6.0/23) is being shared through both Provider 1 and Provider 2
Provider 2 may use a different address (e.g. 192.0.0.1) for yellow network- An address like 201.10.6.17 would match both 201.10.0.0/21 and 201.10.6.0/23
- So, use longest prefix match forwarding
- Router identifies longest-matching prefix (here, 201.10.6.0/23)