Marathon County Uniform Addressing

What is a Zip+4 Code and Why May It Change?

The ZIP+4 code is used and assigned by the United States Postal Service (USPS) for routing purposes. The final 4 digits are not required for delivery of your mail and may change frequently.

There have been several +4 digit changes as a result of the Marathon County Uniform Addressing Program. Whenever a new street or a new block range of addresses are added to the USPS data base, a new +4 may be added. Marathon County does not assign this number; the USPS does.

For example:

Mosinee, WI 54455-1234 --> 54455-6789

A ZIP+4 code uses the basic five-digit code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail, a post office box, or a specific delivery route — meaning the actual path the mail truck would travel could have changed.

Because they are based on delivery routes instead of more permanent boundaries, the last 4 digits of a complete ZIP code can change often. Five-digit ZIP codes also change, but they do so infrequently. Not so for ZIP+4. The +4 on a ZIP code can be changed as frequently as once a month, based on things like how many postal employees are working, who is working what route, etc.

A ZIP code is a postal code used by the United States Postal Service (USPS) in a system it introduced in 1963. An extended ZIP+4 code was introduced in 1983 which includes the five digits of the ZIP code, followed by a hyphen and four additional digits that reference a more specific location. However, initial attempts to promote universal use of the new format met with public resistance, and today the +4 code is not required. In general, mail is read by a multiline optical character reader (MLOCR) that almost instantly determines the correct ZIP+4 code from the address — along with the even more specific delivery point — and sprays an Intelligent Mail barcode (IM) on the face of the mail piece that corresponds to 11 digits — nine for the ZIP+4 code and two for the delivery point.

What is a Zip+4 Code and Why May It Change?