Choice of network space when numbering interfaces with IPv6

Fri Oct 15 23:15:14 UTC 2010

> Date: Sat, 16 Oct 2010 08:51:03 +1030
> From: Mark Smith <nanog at 85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org>
> 
> Hi,
> 
> On Fri, 15 Oct 2010 12:26:13 -0700
> Zaid Ali <zaid at zaidali.com> wrote:
> 
> > SO I have been turning up v6 with multiple providers now and notice that
> > some choose /64 for numbering interfaces but one I came across use a /126. A
> > /126 is awfully large (for interface numbering) and I am curious if there is
> > some rationale behind using a /126 instead of a /64.
> > 
> 
> If you're not going to follow the IPv6 Addressing Architecture, which
> says /64s for everything, then the prefix length decision is
> pretty much arbitrary - there is nothing that special
> about /112s, /126s, /127s or /128s (or /120s or /80s) - they all break
> something in the existing IPv6 RFCs so once you've passed that threshold
> then you're really only choosing your poison. If you're going to go
> down that latter path, I'd suggest reserving a /64 for each link, and
> then using a longer prefix length out of that /64 when you configure
> the addressing, to make it easier if you decided to change back to /64s
> at a later time.

If you listen to the NANOG "debate" on IPv6 on P2P links, you will
discover that the participants (Igor of Yahoo and Rob Seastrom of
Affilias) agreed that the proper way to do this was to allocate a /64
for the link but to configure it as a /127. This was to avoid ping-pong
DOS attacks.

I believe that the session has already been cited, but see Igor's
presentation at:
http://nanog.org/meetings/nanog48/presentations/Tuesday/Gashinsky_LinkNumb_N48.pdf
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
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