[ale] IPv6 local devices with a prefix that may change

[Alex Carver]

This is a purely academic question and thought experiment. It is not 
tied to anything I'm doing specifically or at this time.

Every website/how-to/explainer I've ever run across talks about issuing 
an IPv6 address to each device with the prefix provided by the router 
and that you don't use the equivalent of NAT because IPv6 reasons.

Great, fine, all well and good except no one ever discusses what happens 
when your whole network suddenly has its prefix change.  These sites 
just seem to assume the prefix is static for all time. Well that works 
if you're a company or maybe you're never going to change ISP or move to 
another area. Well that's fine for them but it doesn't really apply 
precisely to me. I've moved several times in the last ten years and 
changed ISPs three times so that prefix would not have been stable.

So suppose this premise:

I write and/or use software for remotely monitoring and controlling 
devices (doesn't matter what they are, IoT, computers, printers, 
anything) and that software, when it starts up, is going to connect out 
to each device it needs to handle. So there's probably a configuration 
file that contains all the IP addresses.

Now, under IPv4 they likely would have all been behind NAT and therefore 
all have private address range IPs which would be stable no matter what 
happened to the NAT device's WAN. But if everyone is now IPv6 and 
getting their prefix from the router a change on the WAN affects 
everyone downstream.

Suddenly my software has an out-of-date configuration because all the 
devices changed out from underneath it.

If one is supposed to do things "The IPv6 Right Way(tm)"/"NAT is bad 
mmm-kay?" using the issued prefixes and such, how do I keep my internal 
network stable so my software can safely work through WAN changes?