6to4 assigns a /48 IPv6 prefix (within 2002://16) to any host or site
with a global IPv4 address. The length of this prefix was chosen to be
the same as the size of a native prefix that was to be allocated to
sites, in order to ease coexistence of 6to4 and native IPv6 and/or
transition from 6to4 to native IPv6. If the normal site prefix
allocation had been /56 or some other value, we would have given serious
consideration to making 6to4 prefixes be the same length.
6to4 is now widely deployed and ships with every major operating
system. It's a bit late to change the length of its prefix. But now
sites that deploy 6to4 will have some disincentive to move to native
IPv6 (or to delay doing so) in that their pain in transition will be
even greater than before.
The /48 prefix length is not just some knob that RIRs or ISPs can turn
at their will. It's a constant that's embedded into 6to4 protocol
implementations in tens or hundreds of millions of computers. That
doesn't mean that /48 can't be reexamined, but it does mean that it's
not the RIRs or ISPs business to be making that decision.
I keep getting the impression that the biggest barrier to the success of
IPv6 is that so many people have screwed with the design of IPv4 that
they think they have the right to screw with IPv6 too.
Keith
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