Cisco Catalyst 3750-E Series, Software Configuration Manual

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Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-08
Chapter 40 Configuring IP Unicast Routing
Configuring IP Addressing
Classless Routing
By default, classless routing behavior is enabled on the switch when it is configured to route. With
classless routing, if a router receives packets for a subnet of a network with no default route, the router
forwards the packet to the best supernet route. A
supernet consists of contiguous blocks of Class C
address spaces used to simulate a single, larger address space and is designed to relieve the pressure on
the rapidly depleting Class B address space.
In Figure 40-2 , classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of
discarding the packet, the router forwards it to the best supernet route. If you disable classless routing
and a router receives packets destined for a subnet of a network with no network default route, the router
discards the packet.
Figure 40-2 IP Classless Routing
In Figure 40-3 , the router in network 128.20.0.0 is connected to subnets 128.20.1.0, 128.20.2.0, and
128.20.3.0. If the host sends a packet to 120.20.4.1, because there is no network default route, the router
discards the packet.
Host
128.20.1.0
128.20.2.0
128.20.3.0
128.20.4.1
128.0.0.0/8
128.20.4.1
IP classless
45749
128.20.0.0