The Choice: IPv4 Exhaustion or Transition to IPv6

Steven M. Bellovin smb at
Thu Jun 28 21:59:33 UTC 2007

On Thu, 28 Jun 2007 17:46:53 -0400
Valdis.Kletnieks at wrote:

> On Thu, 28 Jun 2007 13:08:52 PDT, Bora Akyol said:
> > At a very low, hardware centric level, IPv6 would be a lot easier to
> > implement if
> > 
> > 1) The addresses were 64 bits instead of 128 bits.
> > 2) The extension headers architecture was completely revamped to be
> > more hardware friendly. 
> Wow, a blast from the past.  The *current* IPv6 design was selected
> to a good extent because it was *easier* to do in hardware than some
> of the other contenders.  You think 64 versus 128 is tough - think
> about the ASIC fun and games to support *variable length* addresses
> (not necessarily even a multiple of 4 bytes, in some of the
> proposals. Could be 7, could be 11, check the address length field
> for details. Yee. Hah).

I'm not going to revist all of the design issues; as I said, at this
point IPv6 is what is is.  On that point, you're mostly right; there
were indeed a class of CLNP-derived solutions that were rejected.  That
said, some of us -- including me -- wanted to use the two high-order
bits of the address to select among {64,128,192,256}-bit addresses.
Settling on 128 bits was a compromise between that group and advocates
of a 64-bit fixed-length address.  History since then persuades me that
sticking with 64 bits would have been a very bad mistake.

		--Steve Bellovin,

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