Multiple users have complained that they cannot access servers via RDP. You know that you should check to ensure that the following port is opened:
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Answer Description
Remote Desktop Protocol utilizes port 3389 to function. If this port is closed RDP will not work.
Wikipedia
Remote Desktop Protocol (RDP) is a proprietary protocol developed by Microsoft Corporation which provides a user with a graphical interface to connect to another computer over a network connection. The user employs RDP client software for this purpose, while the other computer must run RDP server software.
Clients exist for most versions of Microsoft Windows (including Windows Mobile but the support has ended), Linux (for example Remmina), Unix, macOS, iOS, Android, and other operating systems. RDP servers are built into the server and professional editions of Windows operating systems but not home editions; an RDP server for Unix and OS X also exists (for example xrdp). By default, the server listens on TCP port 3389 and UDP port 3389.Microsoft currently refers to their official RDP client software as Remote Desktop Connection, formerly "Terminal Services Client".
The protocol is an extension of the ITU-T T.128 application sharing protocol. Microsoft makes some specifications public on their website.
Remote_Desktop_Protocol - Wikipedia, the free encyclopedia 