There are usually 10 to 30 — if you work in a large office it probably has a postcode just for that office, for houses you share with 20-30 or so others. (Very large businesses might have separate postcodes for individual departments, e.g. an electric utility probably has one for handling bills, and another for everything else.)
"What's the postcode please?"
"N1C 4DN"
"And the number?"
"12"
Now they have the whole address. Satnav can take "N1C 4DN" and be very close: https://goo.gl/maps/sGR5XXhmUsLmD2UBA (not the best polygon, should be Handyside Street.)
N1 is innermost, yes, but N2 onwards are sorted alphabetically, and N1C isn't really more "inner" than N1.
(8 if you scroll down)
The only issues are what OC mentioned: some people don’t know a large building (50+ flats) can have several codes, and they are weekly updates because… ::magic dust:: construction!
The worst? Dubai: three inconsistent systems of varying length without any sense, standard, or redeeming features. The city road network is apparently even worse, so I guess those things work hand in hand?
The funniest? One person once joked that people in Ireland were not using postcodes, just the name of the nearby pub, which can get confusing as they often have the same name, so you also have to say the name of the second nearest pub…
I thought was funny, but I wasn’t sure that was a joke. Apparently, that was still true at the time? I saw a lot of discussion about “Introducing PostCodes in Ireland” and avoided those meetings so as not to sound clueless. We used Google Maps for a while during the transition.
Theres a difference between a system and embodied knowledge. I did a lot of work with systems which used Irish addresses early in my career.
"An Post", pre-eircode operated off a traditional hierarchical address system. Where there were "Counties" a real political boundary, "Post towns" which were usually big market towns but the location of a major sorting office, "Localities" (sometimes more than one) which were geographically undefined (we tried) at worst areas and some combination of street and buildings. The hierarchy was not strictly defined, it was a bit hungover.
My own address can be a combination of: <House number> <Street>, <Post town> <House number> <Street>, <Locality>+, <Post town>
Most of these were optional. The "Pub" thing is a testament to how awesome the staff at An Post are at just getting a letter to a door. If you needed to send a letter to "Mary O'Shea" who you knew lived in Kerry and near "Paudie O'Sheas pub" you can bet that if you put "Mary O'Shea, Near Paudie O'Shea's Pub, Kerry" you can bet the letter would get to Kerry, someone in Kerry would know Paudie O'Sheas is in Ventry, Send it on to Dingle and he postie doing the rounds in Ventry would be like "Ah right, thats for Mary" and the only catch being: about 60% of the Female population of West Kerry are probably called some combination of "Mary" and "O'Shea".
I'd imagine logistics companies were dancing for joy with every house having a unique post-code.
So, with users being actively taught not to provide it, of course no (reasonable? :-D) person is going to know it and thus provide it to make it available for use
Further, as mentioned, there is a limited amount of "number" that can be associated with any one postcode (currently 100 for new codes, but some legacy postcodes may have more), so for a long enough street, the postcode will change at some point - for example Chepstow Road, Newport changes from NP182LU to NP182LX at some point. If you have more addresses in a single building than the 100 limit, then the postcode indeed changes within the building.
This is quite useful as the standard way of entering a shipping address is you type your postcode, and then select the exact address from a dropdown, and there's a natural limit on how long you have to scroll to find yours.