We recommend you using port-forwarding. When a computer on the internet sends data to the external ip address of the router, the router needs to know what to do with the data. Port Forwarding simply tells the router which computer on the local area network to send the data to. When you have port forwarding rules set up, your router takes the data off of the external ip address:port number and sends that data to an internal ip address:port number. Port Forwarding rules are created per port. So a rule set up for port 53 will only work for port 53.
More information on port-forwarding you can find here: http://www.portforward.com/help/portforwarding.htm
Please, visit this page, find you router model and follow the onpage instructions: