Re: [v6ops] [EXTERNAL] Re: Scope of Unique Local IPv6 Unicast Addresses (Fwd: New Version Notification for draft-gont-6man-ipv6-ula-scope-00.txt)

Bob Hinden <> Tue, 16 February 2021 03:17 UTC

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From: Bob Hinden <>
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Date: Mon, 15 Feb 2021 19:16:55 -0800
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Cc: Bob Hinden <>, "Manfredi (US), Albert E" <>, Fernando Gont <>, IPv6 Operations <>, "" <>
To: Ted Lemon <>
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Subject: Re: [v6ops] [EXTERNAL] Re: Scope of Unique Local IPv6 Unicast Addresses (Fwd: New Version Notification for draft-gont-6man-ipv6-ula-scope-00.txt)
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Hi Ted,

> On Feb 15, 2021, at 2:05 PM, Ted Lemon <> wrote:
> On Feb 15, 2021, at 4:49 PM, Manfredi (US), Albert E <> wrote:
>> Your mention of birthday paradox depends on how many organizations use ULAs. If not many do, then the likelihood of global uniqueness goes up.
> There are also different uses for ULA. ULA can be used for internal addressing by large orgs, and there there’s potential for overlaps, if for no other reason than that large orgs sometimes merge.
> Another use for ULAs is on home networks. In this case, we don’t expect ULAs to ever need to cross the router. So the set of networks on which home network ULAs need to work is very tightly constrained, and we don’t need to worry about ambiguities.

As a datapoint for this, I own two home style routers from different vendors.   Both generate ULA prefixes automatically.   Vendors seem to have figured this out.


> Another use for ULAs is stub networks. In this case, again we do not expect the ULA to ever make it past the adjacent infrastructure link (the link to which the stub network is attached).
> So chasing after global uniqueness is not necessary in most cases; even in the case where it is possible that there will be conflicts, /global/ uniqueness is not really the issue. In a case where two orgs are merging, the likelihood of a ULA collision, assuming they used a real RNG to generate the ULA, is small, and if it happens, the worst case scenario is that one or both of the orgs need to renumber before they merge. This is not something that’s going to just randomly cause a problem.
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