[128142] in North American Network Operators' Group
Re: Addressing plan exercise for our IPv6 course
daemon@ATHENA.MIT.EDU (Valdis.Kletnieks@vt.edu)
Sun Jul 25 10:29:33 2010
To: Saku Ytti <saku@ytti.fi>
In-Reply-To: Your message of "Sun, 25 Jul 2010 11:40:19 +0300."
<20100725084019.GA3406@mx.ytti.net>
From: Valdis.Kletnieks@vt.edu
Date: Sun, 25 Jul 2010 10:28:53 -0400
Cc: nanog@nanog.org
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
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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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