I just committed this to #OpenBSD -current:
Back in the history of time, IPv4 had classes of addresses. This was widely acknowledged as a failure. At the same time IPv4 classes were declared a
failure, #IPv6 decided to add
them back because using a mac address for IP address configuration was easy.
Now that we have RFC7217 support we can remove this artificial limitation: allow non-/64 prefixes to be configured by SLAAC.
1) Yes, if you have a very small subnet, then you won't have many addresses available for privacy, or collision-avoidance. Duh. You break it, you can keep both pieces.
2) If you have disabled opaque ids (aka, using the old mac-address method), *and* have a smaller than /64 subnet, then you'll only get a privacy address.
@phessler IIRC they were like "you must not give customers less than /56, and a subnet should be /64, but you're not allowed to hardwire it into your routers, as this is just policy, and we might change it"
Did they now change their mind and decide that it's ok to hardwire into routeres?
Anyway, I think we're in the middlebetween two dangers:
vendors hardcoding the /64 and /56 prefix lenghts - we end up in a class-ful nightmare
-ISPs providing less than /56, or even less than /64, per customer
@samis @phessler IIRC the RFCs also contain some recommendations about this. But ISPs can be incredibly cheap. UPC gives /57 to their DS-Lite customers, with a working prefix delegation, and I consider that generous. I've heard of some residential ISPs providing only one /64.
Also, VPS providers will give you /64 if you're lucky, otherwise /128. (online.net is a notable exception, giving each customer a /48).
@phessler @samis yes, but the cable modem needs to allocate a subnet from that prefix on each of its interfaces. If it has 2 interfaces (normal LAN/WiFi, and guest WiFi) it means it needs 2x/64, so the ISP can't get away with giving each customer a single /64. They need to give at least /63, but maybe they'll give /62 or /60. If they could assign a /120 to each of those interfaces, I don't think they would allocate to you anything bigger than /112.