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Opening Ports - If you are using a secure version of VNC, such as VNC Enterprise Edition, you can simply configure your firewall to permit traffic on the port(s) used by the server. If your VNC server is configured to accept connections on VNC Display Number N (equivalent to Port Number 5900+N), then port 5900+N must be configured to be allowed through the firewall. To allow the Java VNC Viewer to be served through the firewall, port 5800+<N> must also be allowed through, or you must configure your VNC server to use the same port for both the VNC and Java Viewer connections, if possible. See also the section on using VNC through a NAT router.
