Re: Addressing plan exercise for our IPv6 course

[Valdis.Kletnieks () vt edu]

On Sun, 25 Jul 2010 11:40:19 +0300, Saku Ytti said:
> On (2010-07-25 17:32 +1000), Karl Auer wrote:
>  
>  
> > The risk of a ULA prefix conflict is for *all practical purposes* zero.
>
> http://www.wolframalpha.com/input/?i=1-((2^40)!)%2F((2^40)^1000000+((2^40)-1000000)!)+
>
> It wouldn't puke nice graph with 'n', it did try, but never finished.
>
> So if there are million assigned ULA's there is 36.5% chance of collision, if
> formula is right.

Bzzt! Wrong, but thank you for playing.

If there exists some screwed-up network design that *interconnects* 1M networks
that are all *advertising* ULAs there's a 36% chance of collision.  It's a
subtle but important difference.  You only care about a collision if (a) you
and some site in Zimbabwe both chose the same ULA prefix *AND* (b) you wish to
set up a private interconnect with them and talk with them *using the ULA
prefix*.  Very important 'and' there.

On the other hand, today if you interconnect *3* private networks that use NAT
you have like a 90% chance of collision.  And yet, people manage to do this all
the time.  So ULAs give a way to make it literally a million times easier - and
THOSE SAME PEOPLE WHO DO THIS WITH NAT ADDRESSES ALL THE TIME ARE WHINING ULA
IS UNWORKABLE.

Geez guys, give me a break.

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