38-7
Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02
Chapter38 Configuring IP Unicast Routing
Configuring IP Addressing
Assigning IP Addresses to Network Interfaces
An IP address identifies a location to which IP packets can be sent. Some IP addresses are reserved for
special uses and cannot be used for host, subnet, or network add resses. RFC 1166, “Internet Numbers,”
contains the official description of IP addresses.
An interface can have one primary IP address. A mask identifies the bits that denote the network number
in an IP address. When you use the mask to subnet a network, the mask is referr ed to as a subnet mask.
To receive an assigned network number, contact your Internet service provider.
Beginning in privileged EXEC mode, follow these steps to assign an IP address and a network mask to
a Layer 3 interface:

Use of Subnet Zero

Subnetting with a subnet address of zero is strongly discouraged because of the pr oblems 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
Step1 configure terminal Enter global configuration mode.
Step2 interface interface-id Enter interface configuration mode, and specify the Layer 3
interface to configure.
Step3 no switchport Remove the interface from Layer 2 configuration mode (if it is a
physical interface).
Step4 ip address ip-address subnet-mask Configure the IP address and IP subnet mask.
Step5 no shutdown Enable the inter face.
Step6 end Return to privileged EXEC mode.
Step7 show interfaces [interface-id]
show ip interface [interface-id]
show running-config interface [interface-id]
Verify your entries.
Step8 copy running-config startup-config (Optional) Save your entries in the configuration file.
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.