The U.S. Postal Service has a ZIP code search functionality on its website that displays every city within the searched ZIP code. A site visitor can also find a ZIP code for a specific address.
The USPS site tool not only displays all of the cities within a ZIP code, but also highlights the correct city to use when mailing to that ZIP code. Other websites such as UnitedStatesZipCodes.org and GetZips.com offer the same ZIP code look-up functionality as the USPS' site. UnitedStatesZIPCodes.org offers printable state ZIP code maps, plus maps for a single ZIP code. The site cautions that some ZIP codes on these maps overlap because a ZIP code can be as small as a city block. With overlapping codes, the site recommends using its ZIP code finder functionality for a specific address rather than a large map. This website also contains the full database of U.S. ZIP codes. A user can download it at no charge as long as it is used for personal purposes. A commercial license has a one-time fee of about $40 as of 2015.
The first digit of the ZIP code designates a broad geographic area of the country with zero for the Northeast and nine for the West. The following two digits pinpoint section USPS centers near population concentrations, with the last two digits referring to small post offices or postal zones in large cities.