<BircGuest-59102936> How can an Apache webserver work internally, have port 80 open, and still not work externally?

It's running Fedora Core 4, and this same installation worked from another internet connection, using pretty much default settings. I moved internet connections and it all works internally, but won't show up externally using my IP. Port 80 is properly forwarded.

Edit: There are no firewalls enabled other than the one in the router itself, and as I mentioned before, the port is forwarded to where it should work.

Thanks for any help in advance.