As far as I know, almost any web site can figure out your IP address, or at least that of your provider (most people get temporary addresses assigned to their local computer by the provider).

My name, zip code and birthday are probably on any number of registration sites, particularly for newspapers and other access-related reasons. But this would imply that the "restricted filter" apparatus has access to all of this.

Neil Gaiman's website mentions this, and says "If you can find the age and zipcode of a real American Resident, you can watch it outside the US."

This implies there's a universally accessible list somewhere. I hacked it using Britney Spears' vital statistics.