Webservice: Use ZipCode to find Longitude Latitude ?

If u try inserting zipcode in google maps, it returns u the exact corresponding location. But, if u want to get the long/lat from it. u can view it in the pagesource/url of browser.

but i need some service which returns long/lat based on zipcode. and than i have to calculate the distance between two sets of long/lat or zipcodes.