If you've ever wondered why Massachusetts ZIP codes start with "0" and California's start with "9," the answer goes back to a 1962 routing decision made inside the U.S. Post Office Department's headquarters in Washington, D.C. The numbering system isn't random — it's geographic, and it sweeps across the country from northeast to southwest. The lowest-numbered active ZIP code is 00501; the highest is 99950. Both have stories worth telling.
The lowest active ZIP code in the United States belongs to the Internal Revenue Service's Holtsville processing center on Long Island. ZIP code 00501 is what's called a "unique" code: it serves no street addresses, no PO Boxes, and no residents. It exists solely to route mail to one organization — the IRS Brookhaven campus that handles tax filings from across the Northeast.
Why "00501" rather than "00001"? When USPS designed the system in 1962-1963, the lowest ZIPs were reserved for U.S. territorial possessions in the Atlantic, including Puerto Rico (which uses the "006-007" range) and the U.S. Virgin Islands (which uses "008"). The "005" prefix was carved out for IRS service centers in the northeast region, with subsequent digits identifying specific facilities.
Several other IRS centers also have unique low-number codes:
If you exclude unique IRS codes, the lowest ZIP code with actual residents is 00601 in Adjuntas, Puerto Rico. Adjuntas is a small mountain town in the central highlands of the island, with a population of roughly 17,000. The town's coffee plantations and mountainous terrain give it a markedly different character from the coastal cities of Puerto Rico, but its ZIP code grants it a small bit of distinction: it's the alphabetically and numerically first place name in the entire U.S. ZIP system.
Each first digit covers a contiguous group of states or territories. The order moves roughly clockwise from the upper-right of the U.S. map:
| First Digit | Region | Example States |
|---|---|---|
| 0 | Northeast & territories | CT, MA, ME, NH, NJ, PR, RI, VT, VI |
| 1 | Mid-Atlantic | DE, NY, PA |
| 2 | South Atlantic | DC, MD, NC, SC, VA, WV |
| 3 | Southeast | AL, FL, GA, MS, TN |
| 4 | Industrial Midwest | IN, KY, MI, OH |
| 5 | Upper Midwest | IA, MN, MT, ND, SD, WI |
| 6 | Central Plains | IL, KS, MO, NE |
| 7 | South Central | AR, LA, OK, TX |
| 8 | Mountain West | AZ, CO, ID, NM, NV, UT, WY |
| 9 | Pacific & Far West | AK, CA, HI, OR, WA, plus territories |
The "0" zone starts in the Atlantic territories and northeast corner of the country, and the "9" zone wraps around to Alaska, Hawaii, and the Pacific. This means the first ZIP code numerically (00501) is geographically in the Atlantic Northeast, and the last (99950) is in the far Pacific Northwest.
The highest active ZIP code, 99950, serves portions of Ketchikan, Alaska, on the southern panhandle of the state near the Canadian border. Ketchikan is a small town of roughly 8,000 residents — best known as the "Salmon Capital of the World" and as a stop for Alaska cruise ships heading to and from Juneau.
Ketchikan actually has multiple ZIP codes (99901, 99928, and 99950 among them). The 99950 code covers a more remote portion of the area accessible only by boat or plane — a fitting bookend for a numbering system that began at an IRS sorting facility in suburban New York and ends in one of the most isolated communities in the country.
It might surprise people that Hawaii — the most distant of the 50 states — doesn't claim the highest ZIP code. Hawaiian ZIPs run from 96701 (Aiea) to about 96898 (which serves Wake Island, technically a Pacific territory). All of Hawaii is contained in the "967" and "968" ranges. Alaska's ZIP codes start at 99501 and continue past 99950 because Alaska was assigned a wider numerical range than Hawaii in the original design — reflecting Alaska's vast geographic spread relative to its population.
Within each region, the numbering continues its sweep:
You won't find every five-digit number in active use. ZIPs are not assigned sequentially without gaps. USPS reserves blocks of numbers for future use, and many codes have been retired when post offices closed, were consolidated, or were renumbered. Out of the 100,000 theoretical 5-digit combinations, only about 41,700 are currently active — leaving more than half the numerical space unused.
The decision to make ZIP codes geographic was deliberate, and changing it now would be enormously disruptive. Address databases, software systems, court records, insurance underwriting models, and even some federal regulations rely on the geographic logic of the first three digits. USPS has explored modernizations — including 11-digit delivery point codes — but the front-facing 5-digit ZIP and its 1963 numbering scheme are essentially permanent features of American infrastructure.
So whether you're sending a letter to 00501 or 99950, the geography is built into the number itself.
Want to look up any specific 5-digit ZIP? Try the free Zip Instant finder for any U.S. address.