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So what do the extra numbers mean? These last 4 digits represent specific delivery routes within delivery areas. This extra detail means an even more precise matching to a more granular level. It could represent a handful of houses on one side of a street, or even a single building that receives a high volume of mail.

As of 1963, zip codes' numbers are determined by a few factors: the area, the regional postal facility and the local zone. The first number of the five-digit code signifies the region which the address is located in, a number that grows from the east coast to the west.

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.

These digits, in conjunction with the first digit, represent a Sectional Center Facility (SCF) or a mail processing facility area. These areas are serviced by the U.S. Post Office SCFs. Note that a single SCF often services multiple three-digit areas.

In general, the first three digits designate a sectional center facility, the mail sorting and distribution center for an area. A sectional center facility may have more than one three-digit code assigned to it.

The first digit in a ZIP code represents a defined group of U.S. states. The second and third digits represent a region in that group, such as a large city. To send mail, only the first five digits of the ZIP code are required.

The lowest ZIP Code number is 00501, unique for the Internal Revenue Service in Holtsville, NY.