Installing behind a firewall

I’ve installed wordpress on a Mac OSX box successfully. It works fine as local host. The box is exposed to the internet. WordPress seems to work fine from the lan, but not from the wan. If I set the site to the WAN address, the site crashes and all outside users get “Can’t connect to localhost” (which is the same error they get when the system is set to the lan IP) and I can’t access it from the lan.

HELP!

I’m trying to move from Joomla to WordPress, and didn’t have these problems with Joomla. Surely WordPress can be set up to work both on the lan and the wan?