We need a router to translate a TCP port for a connection between an inside device and an outside server.

The outside server ( will establish a connection to the inside device ( on TCP port 123, but the inside device is listening on port 234. The router will need to forward inbound traffic destined for port 123 to port 234.

I have found very simple instructions for doing this here, but when I try this I get duplicate IP address errors and the router seems to take over the inside device's IP address. (A "show ip arp" will show the router's MAC address for the inside device.)

ip nat inside source static tcp 234 123

What could I be doing wrong? The instructions are from 2006. Has Cisco changed NAT since then?


  • Are you able to post more of your router config (sanitized)? Or at least the interface configs? Commented Aug 13, 2014 at 14:08
  • That's how it's always worked. When an address is used for PAT, the router "owns" it. And you cannot mix one-to-one NAT with PAT -- it might take the config, but it isn't going to work like you expect.
    – Ricky
    Commented Aug 13, 2014 at 19:34
  • @Ricky I'm new at this, and maybe I didn't clearly describe my scenerio, but that Cisco document linked to above exactly describes our situation. I believe that somewhere between 2006, when the document was created, and now Cisco changed the way one-to-one NAT worked requiring the "no-alias" option to work as originally described in the document. Commented Aug 14, 2014 at 19:09

1 Answer 1


To prevent the router from assuming the IP address you want to port forward from, I discovered that you must add the "no-alias" option at the end.

ip nat inside source static tcp 234 123 no-alias

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