http://en.wikipedia.org/wiki/Network_address_translation 

What I am going to go into is how to configure them on your router. What we’re trying to achieve here is that you’ve got a functioning router and want to direct any incoming requests (SMTP, POP, HTTP – pick one!) to an internal server.

First identify which interface sits on the outside. Then enter global config mode, and issue:

ip nat inside source static tcp <internal ip> <internal port> interface <outside interface> <outside port>

So, for example, if you’re outside interface is FastEthernet1/0, and you wanted to redirect any HTTP requests to your internal web server on 192.168.0.10, you would issue:

phbrouter(config)# ip nat inside source static tcp 192.168.0.10 80 interface FastEthernet1/0 80

What if you’ve got multiple public IP’s? Simply use this instead:

phbrouter(config)# ip nat inside source static tcp 192.168.0.10 80 123.123.123.123 80

Easy, yet effective!

Are you looking to learn more about Cisco equipment? Well, here’s a selection of a few books that I’ve found useful over the years:

Cisco: A Beginner’s Guide

CCNA – Cisco Certified Network Associate Study Guide

Cisco Networking for Dummies

Cisco IOS in a Nutshell – O’Reilly