Port blockage or ????



Win2K3 all MS updates
WinXP Pro clients all MS updates
IE 7 or IE 8

I'm attempting to connect to a selected port on a Windows 2003 SBS server.
My URL configuration is http://123.45.67.89:Port#/MyAppClient/login.aspx.
I'm continuing to get "Internet Explorer cannot display the webpage".
I'm thinking it's a block port (hardware firewall, Windows firewall or other
software) although I can access the appropriate software internally (without
the external IP address).
How can I or is there a way to determine IF a port is block or used etc.?
Is there 'freeware' software to diagnose the problem? At the current time
I'm attempting to determine IF the problem is on 'my end' or 'at the server's
end' or '???'
TIA!
.