Here is a good tutorial on Geotargeting, and/or just a basic introduction to it if you’ve never heard of it.

From Article:

“Geotargeting, or geographical targeting, is a way for website to display content specific for a geographical location. This targeted content can be advertising, latest news, or other examples…”

They state however, that putting in such a filter will impact your performance and slow things down.

http://ekstreme.com/geotargeting/

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