Sounds like some kind of network address translation (NAT) or port address translation (PAT) is happening at your network edge. Do you manage the firewall on the network where the host machine is located? If so, you will need to either map a port/IP address pair to the host machine or setup a mapped IP (NAT) to the machine using the VNC default port 5800. This is not really a good idea as VNC has had vulnerabilities. You should set up VNC through an SSH tunnel and use trusted key pairs to ensure that the connecting client is authorized. Unless you manage the network at the host device end, you likely will not be able to accomplish what you are saying just using VNC. LogMeIN will permit you to remotely access a desktop and is much more secure than VNC.