Re: Non routable IPv6 registry proposal

Fernando Gont <> Wed, 20 January 2021 23:31 UTC

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Subject: Re: Non routable IPv6 registry proposal
To: John Levine <>,
References: <20210120211046.074FE6BC171B@ary.qy>
From: Fernando Gont <>
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Date: Wed, 20 Jan 2021 20:31:24 -0300
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On 20/1/21 18:10, John Levine wrote:
> In article <> you write:
>> The proposal is to reserve a significant block of IPv6 space (e.g.
>> 2002::/16) as non routable address space to be allocated in Class A/B/C
>> sized chunks on a permanent basis either through random assignment or by a
>> new registrar TBD for a negligible one-time fee ($0.10 or less). This would
>> provide significant operational benefits for large enterprises managing
>> complex networks.
> This sounds a lot like ULAs. You generate a 40 bit random number,
> prefix it with FD and that's your own /48. If you have a decent random
> number genarator the chances of your 40 bit number colliding with
> anyone else's are insignificant.  See RFC 4193.

No. Collissions are actually almost guaranteed (birthday paradox).

What the spec says is that *if you grab a handful of ULA networks* they 
(not the entire ULA universe) are quite unlikely to collide.

> I gather the problem in practice is that rather than a 40 bit random
> number, too many people use one that spells something cute or
> memorable in hex.

That's correct -- non-compliant with RFC4193, but still correct.

Fernando Gont
SI6 Networks
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