12.3.3. Firewall Considerations
If you are performing the installation where the VNC viewer system is a workstation on a different subnet from the target system, you may run in to network routing problems. VNC works fine so long as your viewer system has a route to the target system and ports 5900 and 5901 are open. If your environment has a firewall, make sure ports 5900 and 5901 are open between your workstation and the target system.