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Location detecting techniques for IP addresses

For our website it's important to know from exactly which country our visitor is coming from.

I guess the best answer for my question would be the simple geo location feature of current browser systems, which just ask the user if the website is allowed to see his geographic location: http://dev.w3.org/geo/api/spec-source.html

but I don't want to bother visitor with questions, I would love to automatically detects the visitor's location (country should be enough.)

What's the best way to do this?
- what ip database would be the best?
- is the browser header trustable enough to detect his country? (en-us en-en en-ca?)
What's the best solution?