each computer needs to install realvnc server. realvnc can listen to port 5900 - 5999, you can configure each server to listen to a different port number, say 5900, 5901-5903 in server option. then port forwarding your router to that computer, ie port forwarding 5900 to 192.168.0.100, and 5901 to 192.168.0.101, and so on. Finally, from an outside computer, after you start VNC viewer, type in 111.222.111.222:0 to access port 5900, 111.222.111.222:1 to 5901, ...