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Primary state prefixes. 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.

The correct format for a numeric ZIP+4 code is five digits, a hyphen, and four digits. The first five digits represent the 5-digit ZIP Code; the sixth and seventh digits (the first two after the hyphen) identify an area known as a sector; the eighth and ninth digits identify a smaller area known as a segment.

The check digit is calculated by adding up the digits of the zip code and seeing what would have to be added to the sum to get a multiple of 10. For example, the check digit for the zip code 51220 would be 0 (as 5 + 1 + 2 + 2 + 0 = 10), while the check digit for 82125 would be 2 (as 8 + 2 + 1 + 2 + 5 = 18).

?the five-digit ZIP number is a structured code in which the first digit identifies one of ten large areas of the Nation, and the second digit indicates a State, a geographic portion of a heavily populated State, or two or more less populated States.

This is because ZIP code boundaries are based on mail routes and delivery points rather than geographic factors. The U.S. Post Office Department, predecessor to USPS, developed a zoning address system in 1943 as a way to make sorting and delivery of mail easier.

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.

The ZIP code 00001 is one of more than 42,000 five-digit ZIP Codes in the United States. ZIP area 00001 is located in the Dillingham Census Area, state of Alaska.