Do you have a webserver or some other application that is running on your home network that you want to access from outside? This is fairly easy to enable on a typical home router.
On a Linksys router, look under “Applications and Gaming.” Let’s say you have an application called PhotoShare running on a PC or a fileserver that uses port 1000. You need to create an entry on your router. The entry would look something like this:
|Application||Start Port||End Port||Protocol||IP Address||Enable|
When I setup web-access on my Buffalo NAS box, I had to create an entry such as this for it to work.