On 2023-04-18 10:44, Per Jessen wrote:

Well, in my case the router does not reboot often, but the prefix does
change, probably as often as every night (I forgot, I set up logging
once to check and the prefix changed).

That's interesting, I wonder why they do that.  Granted, I've never had
a prefix allocated dynamically, I have always had fixed or my own, so I
could be talking complete rubbish.

The prefix shouldn't change. There's a file called "duid" which the ISP uses to map a prefix to a user. As long as that doesn't change, the prefix shouldn't either. I've had my prefix for over 4 years and I have an ordinary user account. My prefix has survived, at different times, replacing both the modem and the computer I run pfSense on. When I replaced the computer, a couple of years ago, I just copied the config.xml file from the old computer to the new. I only had to adjust the interface names and I was up & running again with the same prefix.