On Thu, 30 Mar 2006 20:43:14 -0600, "Stephen Sprunk"
<stephen(_at_)sprunk(_dot_)org> wrote:
That's why 85% of the address space is reserved. The /3 we are using (and
even then only a tiny fraction thereof) will last a long, long time even
with the most pessimistic projections. If it turns out we're still wrong
about that, we can come up with a different policy for the next /3 we use.
Or we could change the policy for the existing /3(s) to avoid needing to
consume new ones.
I really shouldn't waste my time on this thread; I really do know
better.
You're absolutely right about the /3 business -- this was a very
deliberate design decision. So, by the way, was the decision to use
128-bit, fixed-length addresses -- we really did think about this
stuff, way back when.
When the IPng directorate was designing/selecting what's now IPv6,
there was a variable-length address candidate on the table: CLNP. It
was strongly favored by some because of the flexibility; others pointed
out how slow that would be, especially in hardware.
There was another proposal, one that was almost adopted, for something
very much like today's IPv6 but with 64/128/192/256-bit addresses,
controlled by the high-order two bits. That looked fast enough in
hardware, albeit with the the destination address coming first in the
packet. OTOH, that would have slowed down source address checking
(think BCP 38), so maybe it wasn't a great idea.
There was enough opposition to that scheme that a compromise was
reached -- those who favored the 64/128/192/256 scheme would accept
fixed-length addresses if the length was changed to 128 bits from 64,
partially for future-proofing and partially for flexibility in usage.
That decision was derided because it seemed to be too much address
space to some, space we'd never use.
I'm carefully not saying which option I supported. I now think, though,
that 128 bits has worked well.
--Steven M. Bellovin, http://www.cs.columbia.edu/~smb
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