Classless Routing - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 31 Configuring IP Unicast Routing

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
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 31-2 IP Classless Routing
128.20.1.0
In Figure
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.
Figure 31-3 No IP Classless Routing
128.20.1.0
78-11194-09
31-2, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of
128.0.0.0/8
128.20.4.1
IP classless
128.20.0.0
128.20.2.0
31-3, the router in network 128.20.0.0 is connected to subnets 128.20.1.0, 128.20.2.0, and
128.0.0.0/8
128.20.0.0
128.20.2.0
128.20.4.1
128.20.3.0
128.20.4.1
Host
128.20.4.1
Bit bucket
128.20.3.0
Host
Catalyst 3550 Multilayer Switch Software Configuration Guide
Configuring IP Addressing on Layer 3 Interfaces
31-7