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

Alain Durand alain_durand at
Tue Aug 19 15:28:10 CDT 2008

What I was told is that, yes, the packet get routed through the ASIC, but it
has to go there twice... Hence reducing the pps by a factor of 2 compare to
IPv4. Some vendors had shortcuts that, if the prefix len was < 64, only one
pass was necessary.

Caveat, this may not be true for all vendors or all models of all vendors.

  - Alain.

On 8/19/08 4:22 PM, "Kevin Oberman" <oberman at> wrote:

>> 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
> 128.
> I'd love to get complete verification of the real facts of this.

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