Configure a server behind a Cisco Router (port redirection)
This ‘how to’ isn’t going to go into the hows or why NAT / PAT works on the Cisco router – or in general for that matter. If you’re looking for that – Wikipedia has a good enough write up located here:
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 |
