Cisco IOS Cookbook, 2nd Edition

Allocating External Addresses Dynamically

Problem

You want to dynamically select addresses from a pool.

Solution

You can configure the router to automatically select global addresses from a pool as they are required:

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 pool NATPOOL 172.16.1.100 172.16.1.150 netmask 255.255.255.0
Router(config)#ip nat inside source list 15 pool NATPOOL
Router(config)#interface FastEthernet 0/0
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 FastEthernet 0/1
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 Ethernet1/0
Router(config-if)#ip address 172.16.1.2 255.255.255.0
Router(config-if)#ip nat outside
Router(config-if)#exit
Router(config)#end
Router#

Discussion

This example is similar to Recipe 21.1. The important functional difference is that the internal devices will appear on the outside with different global addresses. The first internal device that makes an outbound connection will get the first address in the range, 172.16.1.100, the next one will get the next address, 172.16.1.101, and so forth.

You configure the range with the ip nat pool command:

Router(config)#ip nat pool NATPOOL 172.16.1.100 172.16.1.150 netmask 255.255.255.0
Router(config)#ip nat inside source ...