The Internet is (by definition) a growing and changing environment. As a result, determining where a specific Internet User is at (geographically) is not a precise technology. As the Internet continues to grow and expand, the ability to track and locate the "geographic origin" of a request is a best guess or an approximation.
That said, we can successfully map most IP Addresses to a general geographic locale with reasonable success. The data behind this mechanism uses GeoLite data available from MaxMind under the Creative Commons Attribution-ShareAlike 3.0 Unported License. It is, essentially, a repository of the geographic locations throughout the world that have been identified by MaxMind as being associated with specific IP Addresses.
Try it yourself by entering a specific IP Address below and clicking "Lookup".