@troglobit also in #ICMPv6 RA / #RFC4861 it seems that again only a prefix plus prefix length is allowed. And not a netmask. Which would seem quite useful for #IPv6 #multicast?

And now I'm also wondering if a host's #IPv6 stack receiving any (routeable) unicast prefix via #SLAAC or #DHCPv6 should maybe just install the matching #embeddedRP #multicast prefix on its own, by default on the according interface?