If you're setting up a listening server, THE VIEWER LISTENS ON PORT 5500.
It's not documented anywhere I saw. Started at a 55,000 port range and narrowed it down to a single port... 5500.
All documentation says 5900 is the magic port.
VNC FYI
As he said, 5900 works fine for the traditional client connect to a server. However, if you setup the client in listen mode, and have the server add that client to it's connected clients. . . that client listens on port 5500. In this setup, the server is basically telling the client "Connect to me". And the client is listening on port 5500 for that server message/command. This setup works well when you don't want to mess with clients' firewalls, but want to be able to control their PC's.