It can be limitations located in many pladces, basicaly your ISP, your DSL, your router, your firewall, your apache config, etc. a similar thread some time ago comes to mind.
Perhaps the thread about port forwarding will provide some info, atleast the online portscanners that is mentioned can be of some help.
You might want to check the advices given there first. If those aren't productive, then we're back to square one.