Using /126 for IPv6 router links

Tim Durack tdurack at
Mon Jan 25 19:02:40 UTC 2010

On Mon, Jan 25, 2010 at 1:01 PM, TJ <trejrco at> wrote:
>> -----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
> low
>> > 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
> subnets.
>> ...
>> > Note: "All we've really done is buy ourselves an 8 to 16 bit improvement
> at
>> > 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! :)
> /TJ

2^128 is a "very big number." However, from a network engineering
perspective, IPv6 is really only 64bits of network address space. 2^64
is still a "very big number."

An end-user assignment /48 is really only 2^16 networks. That's not
very big once you start planning a human-friendly repeatable number

An ISP allocation is /32, which is only 2^16 /48s. Again, not that big.

Once you start planning a practical address plan, IPv6 isn't as big as
everybody keeps saying...
Sent from Brooklyn, NY, United States

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