This gave us such a headache 20 years ago in earlier ecommerce days. One of many edge cases. I'm in Colorado so we always tested with 82082 which covers areas in WY, NE, and CO.

routing. how do we route a physical package to someone, what route does the mail carrier take?. is where the networking term stems from.

you don't really need to know exactly who or where the target house (or IP) is, you just sorta need to know what post office or trucks serve that area (or which networks connect to your target IP / next hop).

thats up to your local post office (or switch) -- send it to them and let them figure it out, which routes and truck to take to get there, like which routes will eventually pass through this locality?

sometimes that's simple and geographical, like a big city and the local post office serves the people around you, 10 miles in any direction, and sometimes it's because there is a mail truck that drives up and down a valley serving people on multiple sides of a state line and in multiple towns.

back in the day, before cars, those routes could be multi-day treks.

also consider that USPS is federal and plays by federal jurisdiction rules, meaning that they don't really care about neat and tidy state or municipal lines -- they're valid everywhere.

This is historical.

The Federal Government really doesn't deal with addresses, roads, etc. That's the job of the states, counties, towns, etc. The USPS, though, needs to know all of that so mail can be delivered.

Via multiple different processes (depending on the entity in charge) the USPS and the entity (state/county/city/town/village/developer/whatever) that comes up with road names/parcels/lots determine the addresses. These addresses get sent to the USPS.

It sounds like the process depends on your specific locality and which government overlays are in charge.

Note that a plat map (the survey that slices land up into chunks) does not necessarily have to include the address.

I lived in a place that was renumbered twice while I lived there, and it wasn't at all obvious - to anyone - as to where the new addresses came from. I asked the county, city, and USPS, and none really could explain how it worked really.

Well, Postal Code is a geographical region but it's not one aligned with legal boundaries.

These are all real scenarios I've lived/have family who has lived:

- Town A contains a region called "Village B" with no distinct government structure. However, Village B has its own post office, so a subsection of Town A has a separate ZIP Code and is delivered mail by this second post office.

- Town C contains two disjoint sub-regions (unincorporated villages) called "Village D" and "Village E." Both have post offices and both have their own ZIP Codes. Some addresses in Village D are delivered mail by the post office in Village E—not the one in Village D.

- Town F has its own post office, but a street in town F receives mail delivered by the post office in Town G.

The practical impact of these is twofold:

- Mail is written to addresses not matching the municipal governmental structure of your town/city. - Mapping apps that rely on USPS data to provided addresses (which is generally good, because it means mail will go to the right place) will show the wrong legal town names for the structure.

Historically, only larger cities had things you might recognize as a postal code. For example: Boston 8, Mass. The current abbreviation for Massachusetts is MA. And some changed - such as NB being Nebraska (and is now New Brunswick) and Nebraska is now NE.

On older letters, you'll see the red abbreviations instead of the more modern ones:

https://en.wikipedia.org/wiki/List_of_U.S._state_and_territo...

And due to historical inertia, very little has changed since the current (5 digit) system was created 60+ years ago.

https://en.wikipedia.org/wiki/ZIP_Code

I don't want to spam this, but I spent too much time writing it to explain this particular case: https://skaldmaps.com/blog/2026/07/zip-codes-are-a-bad-spati...

tl;dr: USPS cares about delivery routes and that's what ZIP codes help with. These happen to make up, more or less, areas. This is close enough an abstraction for many use cases (which is why they are ubiquitous), but pretty horrible once you get into the details.

Postal codes are designed to help with delivering mail so it is less linked to geographic in terms of other things we care about and mostly linked to things that are relevant to mail delivery.