vnc (Virtual Network Computing) is an application that is used to access and manage a computer remotely, a program similar to remote desktop used in Windows.
It is based on an application server to be installed on the computer where you want to access and to install a client application on the PC from which you want to manage the computer remotely, you will be able to ‘be in LAN or the Internet: in both cases should be checked and make sure they are open the ports used for communication between 2 PC, in this case the port 5900/tcp, or the link can not ‘happen.
To handle, open and close the doors of communication with the outside of the PC you vnc use the firewall.

Check the full detail here VNC