cant access via vnc from outside

hi all
i opened two ports on my router for vnc like
tcp 5800 5800
tcp 5900 5900
and give the ip address of the server on which we instaled the vnc.but still i cant access tht server from out side..
any body tell me whts is the probelm or what else i need to do to access the server via vnc..
i m using vigor2800 router and i open these ports in NAT.
wating for reply..