using "reserved" IPv6 space

Karl Auer kauer at
Thu Jul 19 03:57:52 UTC 2012

On Wed, 2012-07-18 at 22:00 -0500, Jimmy Hess wrote:
> Sure... and if  someone says they just happened to toss a coin  128
> times, and got "0" all 128 times, therefore legitimately assigned ULA
> ID is all zeros,    I don't believe them.

Um - 40 times, not 128. The first 8 are set, the last 80 are yours to do
with as you please, and the remaining 40 should be random.

BUT: The whole idea of ULA is that it is for internal use only. If you
want to use 00:0000:0000 as your 40 bits, go for it. Just be aware that
you expose yourself to the risk of pain if, somewhere down the track,
you need to merge your network with someone else who cleverly chose
00:0000:0000 as well.

You can't stop people being making that choice and taking that risk. You
can, however, protect *yourself* by choosing something that is genuinely
random and thus minimising the chance that, come the day when you have
to merge your network with another (including with someone who chose
non-randomly), you will be able to do so relatively painlessly.

I don't understand the professed need for provable randomness. Without a
number *space* to provide context, randomness is inherently
non-provable. The whole point of the randomness of those 40 bits of ULA
infix is that any number is as likely as any other number. Someone,
somewhere, is eventually going to get 10:0000:0000, someone else will
eventually get 20:0000:0000 and so on. And they are just as likely to
get them now as in ten years time.

Because of the likelihood that many people will opt for immediate
convenience at the cost of one-day-maybe-never pain, I would suggest
that you avoid ULA prefixes that *look* non-random to the naked eye. So
if your RNG thows up 00:0000:0001, run it again :-)

Regards, K.

Karl Auer (kauer at

GPG fingerprint: AE1D 4868 6420 AD9A A698 5251 1699 7B78 4EEE 6017
Old fingerprint: DA41 51B1 1481 16E1 F7E2 B2E9 3007 14ED 5736 F687
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