The default port that the Windows Remote Desktop Connection program listens on is 3389. However, you can change to any port you wish; maybe you find public networks block everything except port 80 and 433 (in which you could set it to 433) or you feel changing to a non-default port is better security wise. Whatever your case, you must edit the Windows Registry:
Value: desired port number in Decimal format
Then if you use the Windows Remote Desktop Connection program to connect to the computer, follow the address you want to connect to with a colon and the port number that you changed to; for example 255.255.255.255:433.
Eric Geier is an author of many wireless networking and computing books including Home Networking All-in-One Desk Reference For Dummies (Wiley 2008) and 100 Things You Need to Know about Upgrading to Windows Vista (Que 2007).