The +4 bit of the zip code narrows the location down to a block or street. In the case of apartment/condo buildings, each bank of mailboxes (for each lock that the mail carrier uses) gets their very own +4.
Sample picture: https://postalsupply.net/2023/03/maintenance-and-upkeep-tips...
Do you know of anywhere I can learn more about why this is?
Have they ever thought about changing it so a “postal code” IS a geographical region?
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.
The "city" field in the mailing address is less city and more post office name. USPS is a lot more flexible now, but used to be, if your city didn't have its own post office, your mail would need to be addressed to the city where your post office resided; and even if your city did have a post office, it doesn't mean that's the one that services your delivery point.
Many people don't even live in an incorporated city, so at a municipal level they're just in a state and a county, but not a city. Those people still have to put something in the address field, but that leads to a lot of confusion. Unincorporated towns often have no official basis, and aren't always unique either. Of course, neither are incorporated cities in some states. :P
Changing ZIP codes to be geographical would significantly reduce the usefulness of ZIP codes for their primary use, sorting and delivering mail, so that's a non-starter. Use something else!
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.
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.
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.
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.