Choice of network space when numbering interfaces with IPv6

Date: Sat, 16 Oct 2010 08:51:03 +1030
From: Mark Smith <>


> 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: