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7. Remote Desktop Connection behind a router (Single computer)

A) Configure your router's port forwarding to allow connections on port 3389
B) Now in RDC just type the IP address supplied by your ISP

8. Remote Desktop Connection behind a router (Multiple computers)

A) For each computer you want to remotely connect to you have to configure each computer to listen to different ports to avoid port conflicts by changing the registry key in
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TerminalServer\WinStations\RDP-Tcp\PortNumber

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B) Port Numbers are in Decimal format
Note: To avoid conflict with other programs it is suggested to select a port between 49152 and 65535.

C) Now take that number you've chosen your computer to listen to and configure your router to port forward to that number

D) To connect to that computer your format will now be ISPIPAddress:PortNumber (ex. 222.222.2.8:1234) to connect to that specific computer behind the router

Important: If you have a firewall software running, that too has to be configured to allow communication to the port you opened in your router as well.

Note: The above steps assumes that you're not part of a domain

Reference: http://www.sevenforums.com/tutorials/3922-remote-desktop-connection-rdc-network.html