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As indicated the first time you open the "Geolocation" page, in order to use geolocation, you need need to download a special file first: http://geolite.maxmind.com/download/geoip/database/GeoLiteCity.dat.gz. This file is the GeoLite City database from MaxMind, an accurate database of cities and locations. Download it by clicking on the link, and then decompress it into the /tools/geoip/ directory of your PrestaShop installation.
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You can select or deselect all countries at once by checking the box at the top of the list (next to "Name"). When selecting countries that can access your online shop, make sure to not block any country by mistake, as you would lose all potential sales to its inhabitants!
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