On this page you will find the port number that RDP (Remote Desktop Protocol) uses when you try to connect a to a RDP server with another RDP client. You can use this port number for RDP to troubleshoot firewall and security issues. 
Thu Mar 14, 2013
Remote Desktop Protocol
This forums post will provide you with the RDP port number used in a network with a firewall. It will also help you troubleshoot Remote Desktop Problems.
Find the Appropriate RDP Port Number
For LAN Networks
For WAN Networks
What is RDP?
RDP is a windows terminal server protocol. if you have windows xp professional it already comes installed by default. also, windows 7 professional and ultimate have it installed. if you have windows xp, vista or seven or 8 home edition, it does not come with a RDP server, but it does come with a RDP client.
port 3389 is used for internal (LAN) - for example, you want to remote desktop from one internal computer (example: 192.168.1.100) to another internal computer (example 192.168.1.105)
Port 3390 is for external (WAN) connections - for example, you want to connect to a computer from the internet. example, lets say you are at work and you want to connect to your computer at home using the WAN IP address. then you need to be sure that you have port 3390 allowed either on your firewall or your router at home.
RDP Port Number
RDP (Remote Desktop Protocol) uses port number 3389 for LAN (Internal/Private) traffic and port 3390 for WAN (Internet/public) traffic. if you need to allow RDP into a firewall policy, then these are the ports you need to use for allowing DRP connections or for blocking.
Use this tool to help you troubleshoot problems with RDP Connections. This helpful tool will take you step by step on how to troubleshoot some common RDP failures.
The first common mistake users make when trying to connect to a remote computer is forgot to enable RDP. By default, RDP is disabled, you will need to enable. To enable RDP is very simple, all you have to do is go to the Control Panel > system > Remote Settings > Remote tab, see image below show it should look like:
Under the remote tab you will see three choices:
1. dont allow connections 2. Allow Connections - ALL 3. Allow Connections - ONLY on windows running Network Level Authentication. This option is only available on windows 7 and up.
While 3 is more secured, it means you will have to know about NLA. For troubleshooting purposes, you should select option 2.
After you have confirmed selection #2, try to to connect again to the remote host.
Please note, these setting should be done on the Remote Computer (sever side), not the Host Computer you are using to connect. (client side)
If you confirmed RDP is properly enabled on both the server and the client computers running windows, Then the next obvious problem could be a firewall blocking. Make sure that you have enabled port 3390 to allow RDP traffic to flow. If you are using a laptop running windows for example, go to the firewall settings in the control panel to confirm the settings.