When you use RDP and go out of your own network, you need to specify the "Outside IP address" of the computer you are trying to connect to. You can get the outside address by opening IE on that computer and typing in www.whatismyip.com
That will give you the outside IP address of the system you are trying to connect to.
Now, you need to go onto the router of the network you are trying to connect with and forward port 3389 to the "Inside IP address" of the computer you are trying to connect to. Port forwarding is usually located in the Advanced or Firewall section of your router, usually accessible via a web page interface.
To get the Inside IP address of the computer you are trying to connect to, open a command prompt (Start>Run, type CMD and hit OK).
Now type in ipconfig /all
and you will get a list of your own IP address as well as the default gateway and DNS servers (uusually outside addresses). You will use that IP address when you configure the port forwarding in the router.
I hope this helps. Here is a pic.
Edited by rigacci, 03 January 2008 - 06:43 AM.