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

Ted Lemon <> Sat, 13 February 2021 02:13 UTC

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Subject: Re: [v6ops] Scope of Unique Local IPv6 Unicast Addresses (Fwd: New Version Notification for draft-gont-6man-ipv6-ula-scope-00.txt)
From: Ted Lemon <>
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Date: Fri, 12 Feb 2021 21:13:33 -0500
Cc: Fernando Gont <>, IPv6 Operations <>, "" <>
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To: Fred Baker <>
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On Feb 12, 2021, at 8:55 PM, Fred Baker <> wrote:
>> On Feb 12, 2021, at 4:04 PM, Ted Lemon <> wrote:
>> Global scope is “you can forward it to the default route.”
> No. Global scope, per the RFC  you're quoting, is "Global scope, for uniquely identifying interfaces anywhere in the Internet.”

If you’re referring to RFC 4007, and talking about unicast addresses, then the document is unequivocal on this point: an address is either global or link-local. Those are the only two options. I think here “global” means “there is no context in which this address is, by definition, out of scope.”

I agree that there’s a terminology question here, and perhaps that is a problem to solve, but I don’t see any way to solve it. I think the following things are all true:

ULA addresses are, in principle, VALID in any scope.

They are not, in principle, UNIQUE to a particular link: it’s entirely possible to have two instances of the same ULA referring to different interfaces connected to different links.

In principle, the set of all networks which can route a packet to a particular instance of a ULA /48 MUST be DISJOINT from the set of all networks which can route a packet to some other instance of that ULA /48.

In practice, the randomness of ULAs gives us some reasonable assurance that the principle will hold.

However, users of ULAs that are routed beyond an individual site had better have some policies and procedures in place to make sure that this is true.

Internet backbone routers should never accept BGP advertisements for ULA prefixes.

Sites connecting to the Internet should never, by default, route ULAs northbound of their connection to their ISP.

The last four lines are points of practice, not points of definition of terms.

But the bottom line is that if the term “global” is confusing as it applies to ULAs, it shouldn’t be that hard to clarify what we mean by global.