192.168.x.x is a private address. To access via the internet you will need to set up your router to forward the vnc port to the relevent machine. Depending on the router, you may be able to have diffrent ports for each machine.
Once set up you will access each machine by pointing VNC at your external address and the port number. eg: xx.xx.xx.xx
If you cannot access this function on the router you cannot set this up.