Problem with different network segments [1]

Hello all,

When I am on a same segment of a server I can connect fine with the SUN box. (ex.: my machine is 172.16.X.X and server is 172.16.X.X)
Also works when my machine is 10.2.X.X and server is 10.2.X.X.
But when my machine is 172.16.X.X and server is 10.2.X.X I get XDMCP connection failed.

I can see the server in XBrowser, but when I connect I see a grey dotted screen with an X cursor.

Any ideas??
To get from 172.16.X.X to 10.2.X.X I pas through our firewall, but no ports are blocked for my IP address. I can FTP, Telnet from one to another but can't use XManager. Please Help.

Our firewall is configured so if I have IP 172.16.X.X and want to connect to a 10.2.X.X server the firewall gives me a virtual address for that segment for security reasons (10.2.5.X). XManager doesn't seem to like this.
We configured our firewall so I keep my 172.16.X.X address when connecting to 10.2.X.X. It worked fine.