Classless Interdomain Routing (CIDR)

CIDR

• Networks were once classful
  • RIP v1 was a classful routing protocol
• CIDR permits subnetting
• We can aggregate networks by moving bits to the left
• We can su-divide networks by moving bits to the right

Aggregation

• Collecting a lot of routes

• Aggregation is possible and is easy when you have a lot of networks together that you want to aggregate and the network addressing is contiguous.

• 10.1.0.0/24

• 10.1.1.0/24

• 10.1.2.0/24

• 10.1.3.0/24

• 10.1.4.0/24

• 10.1.5.0/24

• 10.1.6.0/24

• 10.1.7.0/24

-> 10.1.0.0 /21
Host Address Range: 10.1.0.1-10.1.7.254 (2046 hosts)

• One single entry on the router or whatever protocol.
• Group them together with a single subnet mask.