> your external IPv4 address rarely changes

Bad generalization. I'm sure policy about this differs a lot, but my consumer ISP definitely reassigns my home's v4 address periodically. I don't track it closely, but it seems that when my ONT power cycles more often than not it pulls a new v4 address.

Now, basing my privacy/security on this would be bad, but to GP's point, if I was using a static v6 block, not only would this address never change, each device in my LAN would have an extra identifier attached to it. External hosts wouldn't merely be able to identify "my house", but traffic from "my phone", "my kid's switch", and "my spouse's phone" would all have distinct addresses.

Of course, my ISP doesn't do v6 at all, so there's no dilemma :')

That's also a poor generalization, though. Some ISPs rotate customer subnets, and devices can rotate their randomized IPs.

> using a static v6 block, not only would this address never change, each device in my LAN would have an extra identifier attached to it.

This is not true.

IPv6 stack allocates at least 3 addresses:

- Link-local - "Permanent" Address derived from the subnet and MAC - Temporary address that changes several times per day

The default address for new connections is always the temporary address. So IP-based tracking from outside your network will be no better than it was before from one day to the next—the /64 will be the only constant here, just as your router's WAN IPv4 is for v4 connections.

If you had v6, they'd probably also reassign your IPv6 prefix delegation, too.

Also, v6 supports "privacy extensions", essentially randomizing the host portion of the address and periodically rotating it, so it is not accurate to say your address would never change.