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Cisco IOS in a Nutshell, 2nd Edition by James Boney

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Name

ip classless — global

Synopsis

ip classless
no ip classless

Configures

IP classless routing for the router

Default

Depends on the IOS version

Description

This command enables routing based on “classless” addresses. With classless routing, packets can be routed if the router knows a route for a supernet of the addressee. Without classless addressing, the packet is discarded if it arrives at the router and there is no network route for its destination. Assume that a packet arrives with the destination of 10.10.1.5, but we have routes only for 10.10.2.0/24, 10.10.3.0/24, and 10.10.0.0/16. With IP classless routing enabled, the router forwards the packet to the 10.10.0.0/16 network because 10.10.0.0/16 is the best matching supernet of 10.10.1.5.

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