ZIP codes are a system of postal codes used by the United States Postal Service (USPS) since 1963. The term ZIP, an acronym for Zone Improvement Plan,is properly written in capital letters and was chosen to suggest that the mail travels more efficiently, and therefore more quickly, when senders use the code in the postal address. The basic format consists of five decimal numerical digits. An extended ZIP+4 code, introduced in the 1980s, includes the five digits of the ZIP code, a hyphen, and four more digits that determine a more specific location within a given ZIP code.
ZIP codes designate only delivery points within the United States and its dependencies, as well as locations of its armed forces.
Go here to find a ZIP code for a specific address. (When I lived in Richardson, TX, the main post office was located at 433 Belle Grove. Enter this information at the following website to find the zip code for that address. You'll see that the addition of the 4 digits to the basic 5 digits directs your mail to a very specific place.