>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).

I will check this during the week. Thanks