For things like firewall configuration some people will need to know what port the Remote Desktop runs on so they can open that port so they can use it without opening up any extra ports.

Port 3389 is RDP which stands for Remote Desktop Protocol

If you'd like to change this port number:

Locate the following key in the registry:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TerminalServer\WinStations\RDP-Tcp\PortNumber

On the Edit menu, click Modify, click Decimal, type the new port number, and then click OK.
Quit Registry Editor.

NOTE: When you try to connect to this computer by using the Remote Desktop connection, you must type the new port.