When you have a (A)DSL connection like I do, you have to specify your internal IP number in the Network control panel, as well as the router address. For example, 10.0.0.150 and 10.0.0.138. Your real IP address will be different. When you have built a website on your computer (or anything else), you may want to check if what you did was any good. This can be a problem, since your 'outside' IP address is, of course, not 10.0.0.150, but some other number. This can be a fustrating mess. After checking complicated solutions like the Apache conf file and routed, I just found an amazing simple solution!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030407185819509