Using /126 for IPv6 router links

TJ trejrco at
Mon Jan 25 18:01:13 UTC 2010

> -----Original Message-----
> From: Richard A Steenbergen [mailto:ras at]
> Sent: Monday, January 25, 2010 12:08
> To: TJ
> Cc: nanog at
> Subject: Re: Using /126 for IPv6 router links
> On Mon, Jan 25, 2010 at 09:10:11AM -0500, TJ wrote:
> > While I agree with parts of what you are saying - that using the "simple
> > 2^128" math can be misleading, let's be clear on a few things:
> > *) 2^61 is still very, very big.  That is the number of IPv6 network
> > segments available within 2000::/3.
> > *) An end-user should get something between a /48 and a /56, _maybe_ as
> > as a /60 ... hopefully never a /64.  Really.
> > **) Let's call the /48s enterprise assignments, and the /56s home
> > assignments ... ?
> > **) And your /56 to /64 is NOT 1-256 IPs, it is 1-256 segments.
> It is if we are to follow the "always use a /64 as a single IP"
> guidelines. Not that I'm encouraging this, I'm just saying this is what
> we're told to do with the space. I for one have this little protocol
> called DHCP that does IP assignments along with a bunch of other things
> that I need anyways, so I'm more than happy to take a single /64 for
> house as a single lan segment (well, never minding the fact that my
> house has a /48).

Interesting.  I have never seen anyone say "always use a /64 as a single IP"
... perhaps you mean as an IP segment or link?
You are assigned a /64 if it is "known" that you only need one segment,
which yields as many IPs as you want (18BillionBillion or so) - and the
reality is that a home user should get a /56 and an enterprise should get a
/48, at the very least - some would say a /48 per site.

> > **) And, using the expected /48-/56, the numbers are really 256-64k
> ...
> > Note: "All we've really done is buy ourselves an 8 to 16 bit improvement
> > every level of allocation space"
> > *) And you don't think 8-16 bits _AT EVERY LEVEL_ is a bit deal??
> I'm not saying that 8-16 bits isn't an improvement, but it's a far cry
> from the bazillions of numbers everyone makes IPv6 out to be. By the
> time you figure in the overhead of autoconfiguration, restrictive
> initial deployments, and the "now that the space is much bigger, we
> should be reallocating bigger blocks" logic at every layer of
> redistribution, that is what you're left with. So far all we've really
> done with v6 is created a flashback to the days when every end user
> could get a /24 just by asking, every enterprise could get a /16 just by
> asking, and every big network could get a /8 just by asking, just bit
> shifted a little bit. That's all well and good, but it isn't a
> bazillion. :)

There are some similarities between IPv6 and old classful addressing, but
the bit-boundaries chosen were intentionally made big and specifically
factoring in the then-ongoing scarcity (Ye olde Class B exhaustion).  The
scale of the difference *is* the difference.  I am not quite sure what a
bazillion is, but when we get into the Billion Billion range I think that is
close enough! :)


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