Cisco Systems 3750E manual Assigning IP Addresses to Network Interfaces, Use of Subnet Zero, 38-7

Chapter 38 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 addresses. 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 referred 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:

	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]	Verify your entries.
	show ip interface [interface-id]
	show running-config interface [interface-id]
Step 8
	copy running-config startup-config	(Optional) Save your entries in the configuration file.

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:

	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.

	Use the no ip subnet-zeroglobal configuration command to restore the default and disable the use of
	subnet zero.
	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02		38-7