SERVER ON MAC

[4keeper]

I have a problem setting up my mac server.
I'm using The Rumpus management software by Maxum version 5.
I have a static ip coming in thru the the modem then a switch directly to my mac.
So it's live on the internet it's not port forwarded, the router's after the switch connected to my LAN.
I'm able to access the server thru my local network and also connect thru FTP connection thru the browser but not with the straight ip address with http.using port 80.
If I change my port # in the Rumpus App like 8000. I'm able to connect thru the browser with http. My goal is to link my ip with my domain name and not have to tag on a port #.
I have no firewall set up currently my isp says the connection's wide open they have tested it twice and they do not block port 80.
I believe somethings blocking it in the mac OS preferences or the Rumpus app.
I called Rumpus and the settings look fine.
I'm running 10.4.11 OS on my mac.
If anybody's running this config and has some knowledge of the setup on the mac
Please Help!

Thanks
John