Using VNC to control Individual LAN computer through the Internet. I.e. connecting to one computer has to be chosen from few that are on the same LAN.
Install VNC server on each of the LAN computers that you would like to control. Assign to each VNC installation a different port number (do not use the same port for more than one computer).
Example:
Computer 1: IP 192.168.1.10 VNC uses port 5810
Computer 2: IP 192.168.1.20 VNC uses port 5820
Computer 3: IP 192.168.1.30 VNC uses port 5830
*Note: Assigning port number is an option on VNC server configuration menu (look at the upper left side).
Open the ports for the corresponding IP numbers in the Cable/DSL Router's Virtual Server.
Open the same port in the software Firewall of the corresponding computer.
When connecting from the outside type into the VNC Viewer the external IP

ort number.
Example: 68.175.19.50:5820 will connect you to computer 2.
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P.S. This is an authorized copy and paste from this page:
Ultr@VNC - Installation, and Settings