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22-8
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-03
Chapter 22 Configuring IP Unicast Routing
Configuring IP Addressing
Use of Subnet Zero
Subnetting with a subnet address of zero is strongly discouraged because of the problems that can arise
if a network and a subnet have the same addresses. For example, if network 131.108.0.0 is subnetted as
255.255.255.0, subnet zero would be written as 131.108.0.0, which is the same as the network address.
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.
This is an example of partial output from the show running-config privileged EXEC command used to
verify IP subnet zero setting.
Switch# show running-config
Building configuration...
Current configuration : 7454 bytes
!
version 12.1
no service pad
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Perdido1
!
!
<output truncated>
ip subnet-zero
ip routing
no ip domain-lookup
ip domain-name a,b,c
ip name-server 12.10.13.14
<output truncated>
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.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
ip subnet-zero Enable the use of subnet zero for interface addresses and routing
updates.
Step 3
end Return to privileged EXEC mode.
Step 4
show running-config Verify your entry.
Step 5
copy running-config startup-config (Optional) Save your entry in the configuration file.