Using /126 for IPv6 router links

Owen DeLong owen at
Sun Jan 24 16:57:17 UTC 2010

On Jan 23, 2010, at 8:04 PM, Larry Sheldon wrote:

> On 1/23/2010 9:47 PM, Owen DeLong wrote:
>>>> 64 bits is enough networks that if each network was an almond M&M,
>>>> you would be able to fill all of the great lakes with M&Ms before you
>>>> ran out of /64s.
>>> Did somebody once say something like that about Class C addresses?
>> The number of /24s in all of IPv4 would only cover 70 yards of a football
>> field (in a single layer of M&Ms).  Compared to the filling the
>> three-dimensional full volume of all 5 great lakes, I am hoping you can
>> see the vast difference in the comparison.
> Of course--I was asking about the metaphorical message implying "More than we can imagine ever needing".
> I remember a day when 18 was the largest number of computers that would ever be needed.
Do not make the mistake of assuming that just because I support using IPv6
as designed (at least for now) I am too young to remember those things myself.

While I wasn't born early enough to remember the demand for 18 computers
projection of T.J. Watson in the first person, I am quite familiar with the quote
and the environment that fostered it.  I am also familiar with the history of
the internet and it's 8-bit address precursor.

Yes, your point about demand expanding beyond expectation is well taken.
However, I believe that the scale of the IP address space will accommodate
at least a couple of orders of magnitude expansion beyond any anticipated
amount of address space demand. Further, the current IPv6 addressing
scheme does come with a safety valve if people like me turn out to be wrong.
If we're wrong, it will only affect 1/8th of the address space and we can do
something different with the other nearly 7/8ths, possibly setting a 5-10 year
horizon for renumbering out of the first 1/8th into more condensed addressing
schemes so that the original 1/8th isn't wastefully allocated.

Finally, we come to another key difference between IPv4 and IPv6 which
is one of its best features and one of the things that has created the greatest
controversy among legacy IPv4 holders.  There is no IPv6 globally routable
unicast space which is not issued by an RIR under contract with the recipient.
Unlike in IPv4 where the ability to reclaim addresses (whether abandoned,
underutilized, or otherwise) is murky at best, all IPv6 addresses are subject
to a nominal annual fee and a contract which allows the RIRs to maintain
proper stewardship over them.

If I were designing IPv6 today, would I reserve 1/2 the bits for the host
address? No, I wouldn't do that.  However, I do think there is benefit
to a fixed-sized host field. However, the design we have is the design
we have. It's too late to make fundamental changes prior to deployment.
Stack implementations all have the ability to adapt to non-fixed-size
networks if necessary down the road, but, for now, /64s are the
best way to avoid surprises and move forward.


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