Using /126 for IPv6 router links

Mark Smith nanog at
Wed Jan 27 04:53:38 UTC 2010

On Tue, 26 Jan 2010 11:13:22 -0500
Tim Durack <tdurack at> wrote:

> On Mon, Jan 25, 2010 at 11:06 PM, Mark Smith
> <nanog at> wrote:
> > On Mon, 25 Jan 2010 15:15:55 -0500
> > "TJ" <trejrco at> wrote:
> >> I didn't realize "human friendly" was even a nominal design consideration,
> >> especially as different humans have different tolerances for defining
> >> "friendly"  :)
> >>
> >
> > This from people who can probably do decimal to binary conversion
> > and back again for IPv4 subnetting in their head and are proud of
> > it. Surely IPv6 hex to binary and back again can be the new party
> > trick? :-)
> Maybe we can all do this stuff in our head, but I have found removing
> unnecessary thinking from the equation is useful for those "3am"
> moments.
> Given that I am assigning a /48 to a site anyway, and 65k /64s is
> "more than I will ever need", does it really matter if the
> site-specific numbering plan isn't ruthlessly efficient?

The general intent of the /48 allocation is that it is large enough for
nearly everybody, with nearly everybody including all but the largest
of organisations. IOW, it's meant to be "nearly one-size-fits-all", to
try to ensure almost everybody gets as much address space as they'll
ever need at the time of their first request. An addressing plan for
anything less than the largest organsation that doesn't fit within
a /48 or will exceed it fairly rapidly is probably too inefficent.

ps. Remember that I'm one of the ones advocating using /64s everywhere,
so what ever you do, don't use "ruthlessly efficient" to describe my
position - use that for the /126 or /127 crowd ;-)


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