A US ZIP code is a postal code used within the United States to help the United States Postal Service (USPS) deliver mail more efficiently.
The term ZIP stands for Regional Improvement Program. The basic 5-digit number format was first introduced in the United States in 1963 and later expanded to add 4 digits after the dash to form a zip code + 4 digits.
The next 4 digits help USPS group and distribute mail more precisely. Although ZIP codes were originally developed for the USPS, many other express logistics companies such as United Parcel Service (UPS), FedEx (FedEx), DHL, and others utilize ZIP codes to sort and time packages for shipping and cost (freight).
Types of US ZIP Codes
Unique/single high volume address (eg 20505 for the CIA in Washington, DC)
PO Boxes only (e.g. 22313 is PO Box in Alexandria, VA)
army
Standard (all other postal codes)
US ZIP Code Assignment and Mail Classification
The first digit of a U.S. zip code usually represents a group of states in the United States.
The first digit zip code on the map shows their order from northeast to west coast. The first 3 digits of the zip code determine the central mail processing facility, also known as the departmental central facility or "sec center", for processing and sorting mail.
All mails with the same first 3 digits are first sent to the same sec center, where they are sorted according to the last 2 digits and distributed to the local post office. The SEC Center is closed to the public and usually does most of the sorting overnight.
As you can get a map from the first 3 digits of a zip code, the numbers after the first are also generally assigned from east to west. In the map, 0 is closer to white and 9 is much more vivid. It is easy to track the gradient for each region, even with a few exceptions (e.g. the southwestern end of Georgia uses 39XXX, like central Mississippi).
The ZIP+4-digit code is not required, but it helps the post office to additionally sort the mail. ZIP code +4 can correspond to a city block, a group of apartments, or a single high-capacity receiver. It is also common that each PO box number corresponds to a unique zip code +4. Sometimes several PO box numbers are grouped into the same zip code + 4 digit code by using the last few digits of the PO box number. This method is not a general rule, so every PO box must look for ZIP+4.
Some parts of the U.S. are too remote to have a zip code
As you can see on the map, not every place in the US has a zip code. Remote parts of the United States, especially rural areas, don't have enough drop-off addresses to create mail routes.
Without mail delivery, there is no need for a zip code. If you want to get away from the hustle and bustle, these areas are some of the most remote in the country.
US ZIP Code Boundaries
Despite the fact that zip codes appear to be geographical in nature, that's not their intended purpose. They are designed to group mail to allow the USPS to deliver mail more efficiently.
Some zip codes will span multiple states in order to improve the efficiency of mail routing and delivery. In most cases, addresses that are very close to each other are grouped into the same zip code, giving the impression that the zip code is defined by clear geographic boundaries.
However, some zip codes are not associated with geographic regions. For example, all US Navy mail uses one zip code. When zip codes are grouped by geographic location, there is not always a clear shape around the zip code because zip codes are only assigned to a drop point, not the space between drop points. In areas where there are no formal postal routes or mail deliveries, postal codes may be undefined or poorly defined.