Zip Code Structure and Listing

ZIP is an acronym for Zone Improvement Plan. 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 more specific areas, such as small towns or regions of that city. The main town in a region (if applicable) often gets the first ZIP Codes for that region; afterwards, the numerical order often follows the alphabetical order. Like area codes, ZIP Codes are sometimes divided and changed, especially when a rural area becomes suburban.

Geographically, the lowest ZIP Codes are in the New England region such as 02107 in Boston, Massachusetts. The numbers increase southward along the East Coast, such as 10036 New York City, 20500 Washington, DC, 30303 Atlanta, Georgia. From there, the numbers begin increasing heading westward and northward. For example, 40202 is in Louisville, Kentucky, 50309 in Des Moines, Iowa, 60601 in Chicago, Illinois, 75201 in Dallas, Texas, 80202 in Denver, Colorado, 94111 in San Francisco, California, 98101 in Seattle, Washington, and 99950 in Ketchikan, Alaska.

0 = Connecticut (CT), Massachusetts (MA), Maine (ME), New Hampshire (NH), New Jersey (NJ), Puerto Rico (PR), Rhode Island (RI), Vermont (VT), Virgin Islands (VI)
1 = Delaware (DE), New York (NY), Pennsylvania (PA)
2 = District of Columbia (DC), Maryland (MD), North Carolina (NC), South Carolina (SC), Virginia (VA), West Virginia (WV)
3 = Alabama (AL), Florida (FL), Georgia (GA), Mississippi (MS), Tennessee (TN) 4 = Indiana (IN), Kentucky (KY), Michigan (MI), Ohio (OH)
5 = Iowa (IA), Minnesota (MN), Montana (MT), North Dakota (ND), South Dakota (SD), Wisconsin (WI)
6 = Illinois (IL), Kansas (KS), Missouri (MO), Nebraska (NE)
7 = Arkansas (AR), Louisiana (LA), Oklahoma (OK), Texas (TX)
8 = Arizona (AZ), Colorado (CO), Idaho (ID), New Mexico (NM), Nevada (NV), Utah (UT), Wyoming (WY)
9 = Alaska (AK), American Samoa (AS), California (CA), Guam (GU), Hawaii (HI), Oregon (OR), Washington (WA)

Other U.S. territories have codes starting with 9. The next two digits represent the metropolitan distribution point (e.g. 432xx = Columbus OH), and the fourth and fifth digits represents the area of the city (if in a metropolitan area), or a village/town (outside metro areas): 43209 (4=Ohio,32=Columbus,09=Bexley).

Some places with extremely large volumes of mail have their own full ZIP Code, including 81009 for the Federal Citizen Information Center (FCIC) of the U.S. General Services Administration (GSA) in Pueblo, Colorado, and 10048 for the World Trade Center complex in New York, New York (until its destruction on September 11, 2001). The White House has its own secret ZIP Code, separate from the publicly-known 20500, for the President of the United States and his family to receive private mail.

Wikipedia - Zip Code