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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter38 Configuring IP Unicast Routing
Configuring IP Addressing
You can use the all ones subnet (131.108.255.0) and even though it is discouraged, you can enable the
use of subnet zero if you need the entire subnet space for your IP address.
Beginning in privileged EXEC mode, follow these steps to enable subnet zero:
Use the no ip subnet-zero global configuration command to restore the default and disable the use of
subnet zero.
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 38-2, classless routing is enabled. When the host sends a packet to 128.20.4.1, 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.
Figure38-2 IP Classless Routing
In Figure 38-3, 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 128.20.4.1, because there is no network default route, the router
discards the packet.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 ip subnet-zero Enable the use of subnet zero for interface addresses and routing updates.
Step3 end Return to privileged EXEC mode.
Step4 show running-config Verify your entry.
Step5 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