dhcpcd-discuss

Hi,
anothwr discussion here reminded me of a question I alsways had in my mind about assigning prefixes from a delegated prefix block: is the "SLA" a bitfield that gets added to the delegated prefix or is it an integer that gets added to the prefix (base)?
For example, let's ask for a /63 prefix delegation, and the delegating router hands out 2001:db8:0:ff ... for whatever reason, that where it has space for two prefixes. Or is this already an invalid assumption, so delegating routers never allocate on "unaligned" boundaries?
Would then the second prefix 2001:db8:0:ff::PLUS 0:0:0:1:: or is this wrong, because the prefix block needs to be 2001:db8:0:fe:: OR 0:0:0:1?
Best regards, Harald