[hackbinary]

I don't thing that is true.

The client obtains the network prefix from the RA, and then the client tries to generate unique host address.

"" The IPv6 stateless autoconfiguration mechanism requires no manual configuration of hosts, minimal (if any) configuration of routers, and no additional servers. The stateless mechanism allows a host to generate its own addresses using a combination of locally available information and information advertised by routers. Routers advertise prefixes that identify the subnet(s) associated with a link, while hosts generate an "interface identifier" that uniquely identifies an interface on a subnet. An address is formed by combining the two. In the absence of routers, a host can only generate link-local addresses. However, link-local addresses are sufficient for allowing communication among nodes attached to the same link. ""

https://datatracker.ietf.org/doc/html/rfc4862

[w7]

Yes, all implementation that I'm aware of use EUI-64, which requires a /64

https://datatracker.ietf.org/doc/html/rfc7421 is helpful in understanding this.

Basically it explains that SLAAC RFC itself does not define the /64 limitation, but other RFCs that are relevant to network operation do.

""" The addressing architecture [RFC4291] [RFC7136] sets the IID length at 64 bits for all unicast addresses and therefore for all media supporting SLAAC. An immediate effect of fixing the IID length at 64 bits is, of course, that it fixes the subnet prefix length also at 64 bits, regardless of the aggregate prefix assigned to the site concerned, which in accordance with [RFC6177] should be /56 or shorter. """