Hello Dominique
It’s always great to see your posts, and to spend the time looking deeper into the concepts of IPv6!
Thanks for sharing those posts with us.
The answer here is “I don’t know”. This has to do with the policies adopted by each individual vendor and the way the operating system of the device is configured. Typically, if SLAAC is used, a Cisco device will use EUI-64 to generate its IID even for the global unicast address. A Microsoft Windows PC for example, will by default use a random interface ID, but can be configured to use the EUI-64 process. As far as I know Cisco devices cannot be configured to create a random IID. It’s not ideal from a privacy perspective, but there you go.
An effort has been made in IPv6 to disassociate the prefix length from the actual address. Unlike IPv4, where the subnet mask was an absolutely necessary component of the address, the prefix length here is something a little less connected. This is evident from comments I made in this other post responding to another query you had.
The prefix length is always provided by the default router via the RA.
I think the reasoning behind this architecture is to try to make IPv6 as “plug and play” as possible. What I mean is, if everything is set to the default, an IPv6 device plugged in to an IPv6 network, will automatically gain its default gateway and obtain network connectivity without a single configuration from a human being. This philosophy is phenomenal if you think about it, because from now on, new devices (TVs, cameras, refrigerators, washers, dryers, lights, cars, alarm systems, traffic signs, sensors, etc, etc, etc, etc…) will all automatically obtain network connectivity. Do you want to go in and configure each one? Or configure each network to function appropriately? These will all connect by default.
The whole concept behind all of this is to make future networks, which will be networks with devices that don’t have a direct human-computer interface, obtain network connectivity with zero configuration. In order to achieve this “simplicity” it was necessary to create a more complex autoconfiguration concept for such networks. For networks that require more information than is available from the autoconfig, like more traditional PCs, IP phones, mobile phones, laptops etc, such as DNS, and additional DHCP options, the additional functionality of DHCPv6 is added on top of the autoconfig capability.
And as always, I hope this has been helpful!
Laz