It's Ars Tech's turn to bang the IPv4 exhaustion drum

Kevin Oberman oberman at
Tue Aug 19 15:22:57 CDT 2008

> Date: Tue, 19 Aug 2008 14:30:38 -0400
> From: Alain Durand <alain_durand at>
> On 8/19/08 1:50 PM, "sthaug at" <sthaug at> wrote:
> >> In practice, many routers require the packet to go twice in the hardware if
> >> the prefix length is > 64 bits, so even though it is a total waste of space,
> >> it is not stupid to use /64 for point-to-point links and even for loopbacks!
> > 
> > Could you provide some documentation on this? First I've heard about it.
> Ask your favorite router vendor. This has been confirmed to me by at least 3
> major one we use.

Odd. I have asked both of our router vendors and they have confirmed
that they route in the ASIC based on the full address, not just the
first 64 bits. (I believe one of them based on actual testing. I am
suspicious of the other.)

That said, one does use a few bits for something else (port) and does
not load them into the FIB, so I believe they route on 120 bits, not

I'd love to get complete verification of the real facts of this.
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751
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