In your modem setup page, under the Advanced tab should be something like "Port Forwarding", check under the Advanced>Routing perhaps. Anyhow, this will let the modem know any outside connections attempting to connect to your network using port XXX, should have their connection forwarded to whichever IP you set it to. So for example you can run a game server if people attempt to connect to your IP(whatismyip.com) using port 27275.