Cisco IE 4000, Software Configuration Manual

Page: 806 / 1066

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.
Ho s t
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