update to connect to different ports

[Bill S. <fl...@ya...>]

Thanks to all that responded.
situation:
pc ->  firewall > internet > firewall > port forward - 5900 to IP 1, 5901 to IP 2, 5903 to IP3.
Firewall disabled for trouble shooting purposes on all PC machines.  vista/xp/2003 machines. (no ipsec either...)
If I try to use : 
http://x.x.x.x:5902 
I get the prompt for authentication (logs on remote machine verify), once I authenticate, I'm disconnected (by remote machine)
But If I resetup the router/nat to forward 5900 to any destination internal IP (also setup likewise), then it works
http://x.x.x.x:5900
What I end up having to do, is connect to the management interface on the firewall/router (after end user turn it on so I can access),
and reconfigure forwarding (5900) to point to the destination IP in question (192.168.10.x)
This is what I find confusing.
If I can connect on 5900 why can't I connect on 5901 or 2, or 3..etc at the exact same destination?