Providers ISPs provide Internet connectivity ISPs are assigned a large chunk of

# Providers isps provide internet connectivity isps are

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Providers (ISPs) provide Internet connectivity ISPs are assigned a large chunk of contiguous network addresses Organizations contract with an ISP and are assigned one or more network address(es)
Effect of Supernetting on Routing Problem: Recall: Routers (potentially) have an entry in their routing table for each unique network Assigning an organization 256 class C addresses might require 256 routing table entries Assigning an organization 1 class B address would require 1 routing table entry The information that Internet routers must store and exchange increases dramatically
Effect of Supernetting on Routing (cont) Solution: Classless Inter-Domain Routing (CIDR) Collapse a block of contiguous network addresses into a single pair Example: (192.5.48.0, 3) specifies three network addresses: 192.5.48.0 192.5.49.0 192.5.50.0
Effect of Supernetting on Routing (cont) Assume: a small number of ISPs each with a large block of addresses Example: four large ISPs: A: 195.0.0.0 – 195.63.255.0 (2 16 class C addresses) B: 195.64.0.0 – 195.127.255.0 (2 16 class C addresses) C: 195.128.0.0 – 195.191.255.0 (2 16 class C addresses) D: 195.192.0.0 – 195.255.255.0 (2 16 class C addresses)
Effect of Supernetting on Routing (cont) A D C B Z Y X W V
Effect of Supernetting on Routing (cont) Assume: customer X leases the addresses (195.17.0.0, 256) from ISP A ISP A’s routing table: A route to each of A’s subscribers: (195.17.0.0, 256) goes to X A route to each other ISP: (195.64.0.0, 2 16 ) goes to B (195.128.0.0, 2 16 ) goes to C (195.192.0.0, 2 16 ) goes to D Result: CIDR shortens routing tables
CIDR Address Blocks and Bit Masks No need to restrict network numbers to class C addresses No need to use an integer to specify the block size

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• Fall '14
• IP address, Classless Inter-Domain Routing