[cynicalkane]

Standard SLAAC only uses 64-bit prefixes. This is a problem if you want subnets and have an uncooperative ISP handing out 64-bit prefixes. Or maybe you have a /48 or /56 but want a nice readable hierarchy of subnets.

You don't even need 64 bits. IIRC with SLAAC you send NDP messages to the entire broadcast domain. So you need broadcast-domain-sized (i.e., small) device counts on every data-link-layer subnet. So why assign /64? But because some devices won't give up if SLAAC routers aren't there and and some mysteriously default to SLAAC when they shouldn't and in fact some devices only use it, now every bottom level subnet has to be /64. Blech.

This is just one of many IPv6 annoyances that don't even need to exist.

[zamadatix]

/64 is about a lot more than SLAAC, that's just one easily seen place it shows up. Even without SLAAC the /64iveness of IPv6 would still be there. End client subnets being /64 (with the exception of host routes and p2p networks) allow hardware route scaling to be roughly twice as high, allow network operators to never worry about what the client subnet size is (it's /64 and that's big enough for anything, if it's bigger it's an aggregate route, the address is always split in the half for routed part and client part), and allow various tricks like SLAAC assignment methods to work reliably. A /56 gives 256 subnets (and a /48 65,536), if a house needs more than that for hierarchy then something besides the base /64 is wonky.

Uncooperative ISPs will always exist. If the minimum size was /96 they'd hand that out instead of a /64. If there was no minimum size then they'd hand out a /124 or something equally as stupid. For all of the corporatize ISPs and well planned uses, starting at a /64 makes the rest of life easier.

It's the same logic, though often more accepted in this case, as the minimum advertiseable prefix on the internet being a /48. That also has nothing to do with how many theoretical end clients could fit in that and everything to do with what that actually means to equipment, scaling, and implementation planning.