Using /126 for IPv6 router links

Owen DeLong owen at
Tue Jan 26 00:58:29 UTC 2010

On Jan 25, 2010, at 10:50 AM, Larry Sheldon wrote:

> On 1/25/2010 4:45 AM, Richard A Steenbergen wrote:
>> On Mon, Jan 25, 2010 at 09:12:49AM +0000, Andy Davidson wrote:
>>> There are 4,294,967,296 /64s in my own /32 allocation.  If we only ever
>>> use 2000::/3 on the internet, I make that 2,305,843,009,213,693,952
>>> /64s.  This is enough to fill over seven Lake Eries.  The total amount
>>> of ipv6 address space is exponentially larger still - I have just looked
>>> at 2000::/3 in these maths.
>> Don't get carried away with all of that "IPv6 is huge" math, it quickly
>> deteriorates when you start digging into it. Auto-configuration reduces
>> it from 340282366920938463463374607431768211456 to 18446744073709551616
>> (that's 0.000000000000000005% of the original 128 bit space). Now as an
>> end user you might get anything ranging from a /56 to a /64, that's only
>> between 1 - 256 IPs, barely a significant increase at all if you were to
>> actually use a /64 for each routed IP rather than as each routed subnet.
>> As a small network you might get a /48, so that even if you gave out
>> /64s to everyone it would be only 16 bits of space for you (the
>> equivalent of getting a class B back in IPv4 land), something like a
>> 8-16 bit improvement over what a similar sized network would have gotten
>> in IPv4.  As a bigger ISP you might get a /32, but it's the same thing,
>> only 16 bits of space when you have to give out /48s. All we've really
>> done is buy ourselves an 8 to 16 bit improvement at every level of
>> allocation space (and a lot of prefix bloat for when we start using more
>> than 2000::/3), which is a FAR cry from the 2^128 "omg big number, we
>> can give every molecule an IPv6 address" math of the popular
>> imagination. :)
> And it does not account for the factor that I was trying to shine a light on--the it-is-infinitely-huge is at risk of failing due to inventions we can not conceive of.
> Who knew, in the 1940's that every person would be assigned as many as five or more telephone numbers  (exaggeration?  In this house, occupied by two people there are 4 addressable PSTN devices, only one of which leaves the house if one of us does, and there are 6 devices that share an address but could easily have individual addresses, and would if we were using one of the VOIP services).
Sure, but, to put this in perspective, the entire 10-digit NANP could be numbered in a single /64 with several
copies of NANP worth of addresses left over...

NANP: 1,000,000,000 addresses.
/64: 18,446,744,073,709,551,616 addresses

An RIR /12 contains enough end-user /48s to hold NANP and then some:
NANP: 1,000,000,000 addresses
/12: 68,719,476,736 /48s (End User Sites)
/12:      1,048,576 /32s (ISPs) (there are currently less than 50,000 registered phone companies)

> Who knew in the 1980's that refrigerator's would need IP addresses?  (We should not have been surprised, Coke machines did.)
As to refrigerators, probably all the appliances in a house can share a handful of subnets.  A /56 per household
provides for MANY appliance networks as well as separate networks for just about everything else you can
imagine.  Worst case, even if all households end up with /48s the address space is still sufficient.

> And for all the concern about IPv4 exhaustion, what would have happened if the people who fought fiercely against RFC 1918 had won the day?
IPv6 deployment would be a lot further along and we wouldn't have spent nearly so much money
overcoming the pitfalls and problems created by NAT. We wouldn't now have to spend even more
money trying to get past the errant NAT=Security thinking.

> Yes the numbers in IPv6 are huge, no doubt about it.
> But I say, to say "impossible to exhaust" is a fools errand.  Somebody will find a way to exhaust the pool, just to be contrary, if for no currently recognized "legitimate" reason.
No, they're not impossible to exhaust, just pretty difficult.

However, If we see exhaustion coming too soon in this /3, we can always apply a more conservative
numbering policy to the next /3. (And still have 5 /3s left to innovate and try other alternatives).


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