Its really easy to forward a port at your CISCO router 2811. If you have configured your server for simple NAT and looking to forward a port from outside world to inside of your network then you are at a right location. Check the command under given;
ip nat inside source static tcp [Private IP Address] [internal port] [Public IP Address] [external port] extendable
Let me explain the command given above;
If we want to forward port 21 from our external IP address (public) 125.125.208.1 to our port 2931 of internal IP address 192.168.1.208 then we will issue the following command:
ip nat inside source static tcp 192.168.1.208 2931 125.125.208.1 21 extendable
In this way all the inbound traffic coming to our public IP 125.125.208.1:21 will be forwarded to 192.168.1.208:2931
