If your internet access is through a router which does Network Address Translation, you may be able to configure the router to redirect particular incoming ports to particular machines. So you could run WinVNC with a display number of 0 on machine snoopy, and with display 1 on machine woodstock, then set your router to send port 5900 to snoopy and 5901 to woodstock
If you are running a viewer in 'listening' mode, where it accepts connections initiated by the server, it will listen for incoming VNC on port 5500.