you need to open port 5500,5800 and 5900 :
http://portforward.com/english/route...IR-625/VNC.htm
For portcheck, that don't seem to work for local network. I had test for port 15400 for exemple and i know that this port is open and checkport say it is closed... I have test after launching my osaflyff server and yes, it success...
so i think checkport search for an answer when it try to connect on your port, if there is no server connected on this port, checkport will failed, so you neeed to launch vnc il you want checkport to success and see your port open.
It's like script to know if a server is online with an open socket command...if no repons from a server your open socket command will failed...