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ZIP Codes are numbered with the first digit representing a certain group of U.S. states, the second and third digits together representing a region in that group (or perhaps a large city) and the fourth and fifth digits representing a group of delivery addresses within that region.

Delivery addresses do not require ZIP+4 Codes. However, ZIP+4 codes require an address to be first ZIP+4 Codes are based on actual delivery routes. This means that they can change fairly regularly. Some resources state that the USPS can update a ZIP+4 Code as frequently as once in two months.

A United States postal code, for example, might be either five digits or five digits followed a hyphen (dash) and another four digits (12345-1234). If you are validating US postal codes, allow for both conditions. The following rule validates a field that can be five digits, five digits + dash + 4 digits, or blank.

ZIP+4 Codes can change This means that they can change fairly regularly. Some resources state that the USPS can update a ZIP+4 Code as frequently as once in two months. However frequently or infrequently the case may be, it's important to note that they can change.

?While postal code changes do not happen often and we go to great lengths not to change them, updates may be required for a variety of reasons,? said the agency. ?They can happen when converting rural or general delivery addresses to civic/physical addresses.

Due to an increase in population or to the improve postal operations, the US Postal Service® will occasionally add a new ZIP Code or change ZIP Code boundaries.

There are no duplicate ZIP Codes. There are some ZIP Codes that are multi-part polygons, but those polygons are usually adjacent.