Using /126 for IPv6 router links

Mark Smith nanog at
Sun Jan 24 15:45:19 CST 2010

On Sun, 24 Jan 2010 08:57:17 -0800
Owen DeLong <owen at> wrote:

> 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. 

Actually, from what Christian Huitema says in his "IPv6: The New
Internet Protocol" book, the original IPv6 address size was 64 bits,
derived from Steve Deering's Simple Internet Protocol proposal.
IIRC, they doubled it to 128 bits to specifically have 64 bits as the
host portion, to allow for autoconfiguration.

> 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.
> Owen
> > -- 
> > "Government big enough to supply everything you need is big enough to take everything you have."
> > 
> > Remember:  The Ark was built by amateurs, the Titanic by professionals.
> > 
> > Requiescas in pace o email
> > Ex turpi causa non oritur actio
> > Eppure si rinfresca
> > 
> > ICBM Targeting Information:
> > 	

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