I've managed to enable port-forwarding on my Cisco 877 but I wasn't able to find a way to specify a single accepted IP from the outside. Any link to a Cisco documentation or commands to type in IOS are welcomed.
Thanks by advance.
"NAT" in and of itself won't disallow anyone. It will do translation. It sounds like you have set up a port forwarding to an internal device, but want to allow only 1 external device to use that particular port forward?