Manual port mapping, also called port forwarding, requires that you enter a fair amount of seemingly arcane information to get a computer on your local network receptive to incoming traffic from the greater Internet via your router. With respect to screen sharing, you use port mapping in two cases:
For any screen-sharing service, including Back to My Mac and screen sharing over iChat, if your router doesn't support automatic port mapping (as noted in Which Kind of Configuration?, earlier in this section).
For screen sharing with a direct connection.
Because all of Apple's services have fixed ports for remote access, if you use manual port mapping, you can run a given service on only one computer at a time on your network. ...