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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 31 Configuring IP Unicast Routing
Configuring IP Addressing on Layer 3 Interfaces
Beginning in privileged EXEC mode, follow these steps to assign an IP address and a network mask to
a Layer 3 interface:
Use the no ip address interface configuration command to remove an IP address or to disable IP
processing.
This example shows how to configure an IP address on and enable Gigabit Ethernet interface 0/10:
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface gigabitethernet0/10
Switch(config-if)# no switchport
Switch(config-if)# ip address 10.1.2.3 255.255.0.0
Switch(config-if)# no shutdown
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.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Enter interface configuration mode, and specify the Layer 3
interface to configure.
Step 3
no switchport Remove the interface from Layer 2 configuration mode (if it is a
physical interface).
Step 4
ip address ip-address subnet-mask Configure the IP address and IP subnet mask.
Step 5
no shutdown Enable the interface.
Step 6
end Return to privileged EXEC mode.
Step 7
show interfaces [interface-id]
show ip interface [interface-id]
show running-config interface [interface-id]
Verify your entries.
Step 8
copy running-config startup-config (Optional) Save your entries in the configuration file.
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.
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