802
Configuring IP Unicast Routing
Configuring IP Addressing
DETAILED STEPS
Use the no ip subnet-zero global configuration command to restore the default and disable the use of subnet zero.
EXAMPLE
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# ip subnet-zero
Switch(config)# end
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 98 on page 802, 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 98 IP Classless Routing
In Figure 99 on page 803, 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.
Command Purpose
1. configure terminal Enter global configuration mode.
2. ip subnet-zero Enable the use of subnet zero for interface addresses and
routing updates.
3. end Return to privileged EXEC mode.
4. show running-config Verify your entry.
5. copy running-config
startup-config
(Optional) Save your entry in the configuration file.
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
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