Puzzeling about IPv6

夜神 岩男 supergiantpotato at yahoo.co.jp
Sat Nov 19 19:32:33 UTC 2011

On 11/20/2011 04:07 AM, Matthew Seaman wrote:
> On 19/11/2011 18:47, 夜神 岩男 wrote:
>>> Oh, and given you've got 64bits to play with, so long as your random
>>> numbers are up to scratch no need to worry about collisions.  You'ld
>>> need to be assigning millions of addresses before you ran into that
>>> problem.
>> Not to be an ass and this is likely a decade too early, but... this is
>> direct echoes of what I heard 20 years ago.
>> Does systematic thinking belong in /32+ IPv6 addressing or is it in fact
>> safe to just random it all away willy-nilly?
> Look at http://en.wikipedia.org/wiki/Birthday_paradox
> With 64bits of host address space in a typical IPv6 network, you would
> need to be allocating 6.1 million addresses to have a 1 in a million
> chance of a collision.  You'ld need 5.1 billion addresses for a 1 in 2
> chance of a collision.  If you get a collision in a typical network of
> maybe several hundred machines, then suspect your random number
> generator before anything else.

I would appreciate the numbers more if we were talking in terms of 
numbers of machines, as we were in the late 80's, but we're not. Now 
everything has an address. With virtualization (which is a trend I tend 
to buck, but is a prevalent force) it is currently normal for a single 
machine to host tens or hundreds of IPs. With the mobile environment and 
some concepts to simplify mobile-but-hubbed/homed devices even those 
devices can inherit several IPs each. Is it not inconceivable that 
complete ignorance of numeric paritioning could run us into weird places 
quicker than we expect once again?

For example, a random assignment gives me something close to the < /8 
space of the low end of my range and/or another pre-assigned address 
region which was initially intended for a single machine -- until that 
machine and its IP space became all cloudy like (the way 1st year 
drop-out CIO's are getting sold on today). Now is this range enough, and 
is the resolution overhead worth it in the future (10+ years of us 
thinking IP ranges are freely available enough to just ranomd assignment 
away) to push the next bajillion addresses to the same machine/cluster 
(as it will no doubt evolve into at some point) to a totally separate 
random remaining range once the available random addressing block is 
used/randomed away?

The fact you cite the birthday paradox is interesting, as it predicts 
that collisions are highly likely given the way we've grown to think 
that every device should be multiply homed within a massively 
multi-homed cluster and that IP assignments are totally costless today.

Or perhaps everyone is fatigued enough by the IP4 experience to just 
wish this away.

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