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

Fred Baker <> Fri, 12 February 2021 22:37 UTC

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From: Fred Baker <>
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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)
Date: Fri, 12 Feb 2021 14:37:26 -0800
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To: Fernando Gont <>
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> On Jan 5, 2021, at 5:20 PM, Fernando Gont <> wrote:
> ULAs are formally part of the GUA space. However, the characteristics of ULAs do not seem to match the definition of global scope from RFC4007 (IPv6 Scope Addr Architecture). ULA seem to have a scope of scope(link-local) < scope(ULA) < scope(GUA).

I think I might describe it using the phrase "routing scope" or something akin to it. An address, any address, is usable within the range it is advertised to in routing, router advertisements, and so on - and it is unreasonable to expect routers to actively limit that apart from some form of configuration to do so. So to my mind, statements to the effect that ULAs should or should not be limited to (or from) some domain are not instructions to routing implementations per se, but instructions to people that configure BGP accordingly. Link-local addresses are confined to a given LAN not because someone said so in an RFC, but because routing implementations do not advertise a certain prefix off-LAN, and ULAs are confined to a given domain not because someone wrote it in an RFC, but because routers are not configured to (are configured to not) advertise them to external BGP peers and (hopefully) BGP peers refuse them if inappropriately advertised to them.

We're trying very hard to construct a useful definition of "site-local" without saying so, and it might be worthwhile to actually say so.