Cisco IOS Cookbook, 2nd Edition

Configuring Basic NAT Functionality

Problem

You want to set up Network Address Translation on your router.

Solution

In the simplest NAT configuration, all of your internal devices use the same external global address as the router’s external interface:

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#access-list 15 permit 192.168.0.0 0.0.255.255
Router(config)#ip nat inside source list 15 interface FastEthernet0/0 overload
Router(config)#interface FastEthernet0/2
Router(config-if)#ip address 192.168.1.1 255.255.255.0
Router(config-if)#ip nat inside
Router(config-if)#exit
Router(config)#interface FastEthernet0/1
Router(config-if)#ip address 192.168.2.1 255.255.255.0
Router(config-if)#ip nat inside
Router(config-if)#exit
Router(config)#interface Ethernet0/0
Router(config-if)#ip address 172.16.1.5 255.255.255.252
Router(config-if)#ip nat outside
Router(config-if)#exit
Router(config)#end
Router#

Discussion

In this example, the router will rewrite that address of all of the internal devices whose IP addresses are in the range 192.168.0.0/16. When these internal devices connect to devices on the outside of the network, they will all appear to have the same source address as the external interface of the router, 172.16.1.5.

This example actually includes two internal interfaces and one external. You designate the internal interfaces with the ip nat inside command. You can have as many inside interfaces as you like:

Router(config)#interface  ...