Classless Interdomain Routing

Classless interdomain routing (CIDR) is used in Internet Border Gateway Protocol (BGP) routing to reduce the number of Class C routes coming from an autonomous system. For example, ISPs can allocate blocks of 4, 8, or 16 Class C networks to a customer and use only one route for reachability. In Figure 6-3, the client has a CIDR block of four Class C networks, but Router A announces only 208.10.8.0/22 to the ISP router.

Figure 6-2 Route Summarization

192.168.16.0/24

192.168.17.0/24

192.168.16.0/24

192.168.17.0/24

192.168.20.0/22

192.168.18.0/24 192.168.19.0/24

192.168.20.0/24 192.168.21.0/24 192.168.22.0/24 192.168.23.0/24

192.168.20.0/22

192.168.18.0/24 192.168.19.0/24

192.168.20.0/24 192.168.21.0/24 192.168.22.0/24 192.168.23.0/24

Figure 6-3 Classless Interdomain Routing (CIDR)

Internet Service Provider a

208.10.8.0/22 CL Router A

208.10.8.0/22 CL Router A

0 0

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