% Expect to see routers being optimized that will only route % the upper 64bits of the address, so you might not want to do % anything smaller than that.
This, if it happens, will be exactly opposed to the IPv6 design goal, which was to discourage/prohibit
hardware/software designers from making presumptions or
assumptions about the size of prefixes and HARDCODING them
into products.
That is a foolish design goal demonstrating lack of understanding on how large 2^64 space is.
A better design, for example, is to use only upper 16 bits (note that 100,000 routes of IPv4 is already overkill) at the core backbone, which makes high speed backbone routers not very expensive and faster.
Though multicast and QoS needs more bits, they, anyway, need separate mechanism.
Masataka Ohta
