>On Web sites where the user has to enter his postal code or zip code in his profile, this is often used for the proximity search. On a Web site I am hosting, I need to provide that type of search. So, a user could search for people around him within a 50km radius. I believe this is often related to a numeric ID associated to the postal code or to the zip code. Otherwise, the only other thing I would see would be to use the IP address, but then, this is no longer valid is the user is traveling.
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>So, basically, is there some kind of table that exists for that or how does that work exactly?

Hi,

You'll obviously need a database that maps post or zip codes to a coordinate system (probably lat/long). Available from several sources depending on the countries you are interested in. Algorithems for compuing distance between coordinate pairs are also widely documented.

Here's one possible provider:http://www.geopostcodes.com.
Pricing seems reasonable and I liked the fact that 'Vatican City' is free (but there's only one postcode).

HTH,
Viv