According to the US Postal Service's current website, the lowest ZIP Code number is 01001 in Agawam, Massachusetts, and the Highest ZIP Code number is 99950 in Ketchikan, Alaska.

However, it should be noted that two special examples exist. The zip code 00501 is valid for Holtsville, New York, but is used exclusively for the US Internal Revenue center there. And while not a U.S. state, the zip code 00601 is valid for Adjuntas, Puerto Rico.

The ZIP code is America's version of the postal code which was first used in Germany in 1941. In 1963 America adopted the postal code, but named it Zone Improvement Plan code or ZIP code for short. The Zip code is used in all 50 states as well as the American territories.

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