Re: Non routable IPv6 registry proposal

David Farmer <> Thu, 11 March 2021 19:19 UTC

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From: David Farmer <>
Date: Thu, 11 Mar 2021 13:19:11 -0600
Message-ID: <>
Subject: Re: Non routable IPv6 registry proposal
To: Keith Moore <>
Cc: IETF-Discussion Discussion <>
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On Thu, Mar 11, 2021 at 10:49 AM Keith Moore <>

> On 3/11/21 5:22 AM, Nico Schottelius wrote:
> Another question I have is whether such ULA allocations
> will realistically remain local.
> ULAs are unlikely staying local, as we have seen with radio networks in
> Germany. Tunnels are being used to interconnect remote cities and
> non-collision (not necessarily public routing) are a primary concern.
> Despite the name, there's no reason that ULAs should stay local.   As long
> as they are properly chosen, it's perfectly reasonable to route them
> privately between cooperating networks, and IMO this is part of their
> design.   One of the problems with RFC 1918 addresses in IPv4 was that
> enterprises had a need to route traffic between networks each using that
> space.   The resulting address collisions generally required explicit NAT
> configurations to work around, and these were failure-prone and difficult
> to manage.  ULAs were intended in part to remedy this problem.
> Keith
The "L" for Local isn't intended to have a strict definition of Local.
However, similarly, the "U" for Unique isn't intended to have a strict
definition of Unique either, especially a mathematical definition of

You can easily interconnect thousands or even tens of thousands of ULA
prefixes without much chance of an address collision, as long as the random
assignment process is actually used. Whereas, if you try to interconnect
billions of ULA prefixes, you will probably start running into the birthday

So the interconnection of ULA prefixes, the route-ability of them, is not
intended to be unlimited. There are limits to the number of ULA prefixes
that SHOULD be interconnected to each other; nevertheless, this limit is
extremely generous for the intended use cases.

If you disregard the intended use cases and use them outside the intended
use cases, then address collisions could become an issue.

David Farmer     
Networking & Telecommunication Services
Office of Information Technology
University of Minnesota
2218 University Ave SE        Phone: 612-626-0815
Minneapolis, MN 55414-3029   Cell: 612-812-9952