Honest question, how does privacy come into play here? If you're given a /64, even if you change the last 64 bits, isn't it trivial for someone to assume everything from the first 64 is you?
Yeah. It is a trivial assumption. In my experience with Comcast Residential internet, one's IPv6 prefix remains the same for as long as one's IPv4 address, which is to say that they remain the same forever.
Comcast hands out allocations as wide as /60, but even this doesn't help much with privacy; if you're being unusually proactive with your network renumbering, that's only four bits of entropy that you're adding to your identifiers. :)
1. The /64 is the same for your whole local network. Granted that at home that is usually not many devices, but it's almost certainly more than one.
2. The /64 changes when you change networks, and unless you have a static IP address it will change for your home network too. On the other hand, if the low 64 bits is derived from your MAC address, it never changes (unless you replace your NIC of course.)
> The /64 is the same for your whole local network.
This means that -at best- IPv6 "Privacy Extensions" give advertisers no more information than they get today with non-Carrier-Grade IPv4 NATs. That's not a big win, in my book. :/
