ipv6 addressing - non-routable equivalents?

[Antonio Querubin]

On Sat, 30 Sep 2000, Bill Manning wrote:

> 	  If you have:  201:0600:0004:80cf::/48 and (remember) the
> 	  bits from 65-128 are "reserved" for your MAC or e164 address,
> 	  then you have /49 to /64 to carve up as subnets.
> 	  16 delegation points, e.g. the functional equivalant of an
> 	  IPv4 /16.  Does that help?

Is it really required that we use the MAC address in bits 64-128?  Ie.
what prevents someone from just starting with some arbitrary number in the
subnet field?  Or for that matter what really prevents subnetting beyond a
/64?