The remote desktop protocol (RDP) is a protocol used by remote desktop to connect to a windows machine remotely. Although it is a Microsoft protocol, the client is available for Linux, Mac, ipad etc.
RDP access on a computer is enabled through the properties of “my computer” or “this pc” and users can be specifically allowed or denied access.
To access a computer on the network from outside, it is necessary to forward a port from the firewall to the server computer. So for example if the server on the network has a private address of 192.168.0.166 and the public IP address of the firewall is 18.104.22.168 then a rule needs to be placed in the firewall to forward the port. The listening port is by default 3389 but this can be altered and the firewall can be configured to forward a different port. For example 2299 on the internet side could be forwarded to 3389 on the LAN side. In this example, to connect to the server you would type in to remote desktop connecton 22.214.171.124:2299
Although using RDP for external access is convenient, easy and cheap it should be thought of as unsafe and should not be done. A large percentage of successful ransomware attacks are thought to be delivered through RDP either through misconfiguration, bugs or brute force attacks.
Remote access from outside the network should always be done via the additional protection of a VPN