Re: Non routable IPv6 registry proposal

Brian E Carpenter <> Wed, 20 January 2021 21:32 UTC

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Subject: Re: Non routable IPv6 registry proposal
To: Nick Hilliard <>, Phillip Hallam-Baker <>
Cc: IETF Discussion Mailing List <>
References: <> <>
From: Brian E Carpenter <>
Message-ID: <>
Date: Thu, 21 Jan 2021 10:32:26 +1300
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> if you don't need 
> both, then ULA should work fine.

More completely: if you don't need both, *or* if you are willing to risk the unlikely inconvenience of renumbering if your ULA network ever merges with, or directly interconnects with, another ULA network that by chance has the same pseudo-random prefix, then ULA should work fine. The birthday paradox part of this is discussed in section 3.2.3 of RFC 4193.

RFC 4193 also reserves, but does not specify, a range of such addresses (usually known as ULA-C) that could in theory be centrally registered, if people don't accept the birthday paradox risk. That was the topic of the recent discussion that Nick mentioned. So there is no need to assign anything new. The only issue is how to fund such a registry and guarantee it indefinitely.

As for NAT, it remains totally unnecessary for IPv6 (see RFC 4864). We do have a half-acceptable solution (i.e. it mitigates some of the damage done by NAT) as an *Experimental* RFC 6296.

   Brian Carpenter

On 21-Jan-21 09:58, Nick Hilliard wrote:
> Phillip Hallam-Baker wrote on 20/01/2021 20:06:
>> 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 idea was the subject of a recent discussion on 6man, subject 
> thread: "Re-Launching the IPv6 ULA registry".  The original email was here:
> There were several aspects which cropped up, but the core issues seem to 
> be whether the end user needs both address permanence and the 
> requirement for interconnection to third parties.  If you need both of 
> these, then registered addresses are a good idea; if you don't need 
> both, then ULA should work fine.
> There are options out there for getting formally registered address 
> space at modest cost.  It's not 10c once off, but it's not going to 
> break the bank either.
> The economics you're proposing may need a bit more consideration, 
> especially given that registries need long term stability, both 
> financial and from the point of view of governance.
> Nick