Just to chime in agreeing with sibling comments, the issue is when it's not about just your house and you're deep into self-hosted stuff with a lot of different properties and businesses other people's houses all sharing resources. Without a lot of coordination and consideration, which in practice doesn't really happen easily given the adhoc nature such things tend to organically develop out of, and all the random stuff that wants specific addresses at least for setup, it actually gets pretty easy to run into collisions. Allocations typically are definitely inefficient in many respects but also made sense in the context they were first done and of course can be a certain amount of effort to change.

In an ideal world I sorta feel like "IPv6" should have been more along the lines of <12 octet prefix>:<IPv4>, everyone gets a prefix or set of prefixes that they actually own and are consistent worldwide, or at least only change when geographic location changes, and then can just have the entire 32-bit IPv4 space for LAN however they want. Then you only have to care about prefix between LANs and it could all be extremely automated, internally you only need to use something that looks like IPv4 with the network hardware transparently able to handle prefixes for WAN. Backwards compatibility story would be a lot more straight forward too. Oh well.

@kotaKat: that's a great idea and I don't know why I forgot I played with that like 15 years ago. Really handy as a backup space that almost certainly won't collide with any commercial hardware at least.

But one of our vendor networks uses 10.32.x.x/32 for various radio gateways, and the radios themselves all locally expose (this is where I give away too big a clue as to what I do) 10.0.0.101 as a management address that emits important link status data.

So you can imagine what a godawful bùrach everyone's routing tables are.