Comment by andix

> Isn't that the default for modern routers?

I assumed so because it's the obviously correct thing to do, but I haven't used a SoHo router in so many years, so I don't know.

> Random ULAs are also great for LAN-to-LAN VPNs, because there aren't any private IP overlaps.

Yep! Extremely low chance of prefix collision is their most important purpose. I've been part of several corporate acquisitions, and it inevitably took 6->18+ months to do enough of the IPv4 renumbering and server-discommissioning [0] process to begin to merge the LANs of the merging companies. Had they used ULA addressing, the odds of having to do that massive amount of work would be effectively zero.

And if collisions aren't something you give any shits about, nothing stops you [1] from setting your ULA to fd00::/48 and advertising fd00::/64 on your LAN for static IP addresses that are notably shorter than you can get with IPv4!

[0] ...because there's so little space in the 1918 range...

[1] ...well, unless your router's configuration system doesn't let you specify your ULA prefix...

ULA are useless for most practical purposes because of the source address selection rules.