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[IPv6]Re: [v6ops] Re: New draft: "The IPv6 Loopback Address Prefix"

Gert Doering <gert@space.net> Wed, 26 November 2025 07:27 UTC

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Date: Wed, 26 Nov 2025 08:26:49 +0100
From: Gert Doering <gert@space.net>
To: Mark Smith <markzzzsmith@gmail.com>
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References: <CAHw9_i+b=uZozstCAm1Kr52Pj-_Y_aCndHc0e703rMUr9va=iA@mail.gmail.com> <CAO42Z2xsGuZ+5V8SadxRRkeeL7owm35F9MO8owAcWwfi9Q6nFw@mail.gmail.com> <F04C4F2A-C664-4B68-875B-C4C6CF3B6C64@gmail.com> <CAO42Z2x85B3Cn87QZQqhDef28Pfp_ukWqNO71Ucg=Jyut_NEkA@mail.gmail.com>
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CC: Geoff Huston <gih902@gmail.com>, IPv6 <ipv6@ietf.org>, IPv6 Operations <v6ops@ietf.org>
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Subject: [IPv6]Re: [v6ops] Re: New draft: "The IPv6 Loopback Address Prefix"
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Hi,

On Wed, Nov 26, 2025 at 11:03:20AM +1100, Mark Smith wrote:
> On Wed, 26 Nov 2025 at 10:19, Geoff Huston <gih902@gmail.com> wrote:
> > thanks Mark - except that you should  substitute the /32 to a /96 as I got confused between my left and my right!
> >
> 
> One of my use cases, which I think I hinted to but didn't specifically
> state, was multiple loopback interfaces on a node.
> 
> On Cisco routers you can create multiple loopback interfaces. I've
> done that so that I could have say a management loopback interface and
> address, a BGP session loopback interface and address and then other
> loopback interfaces and addresses for other functions such as an
> L2TPv2 end point, RADIUS service client address etc.. Those loopback
> interface addresses were all announced into the routing protocol (or
> not depending on function).

This is a very different thing from what the draft proposes to establish.

You are basically deliberately choosing and assigning routable addresses
to "non physical" device interfaces, for externally-reachable functions
(and this is a very reasonable thing to do, so "all the router people"
do it, and "all the anycast service people" do it as well...).

The draft at hand proposes to extend the single-ip loopback interface 
to a prefix, with well-known IPv6 addresses "these are available on every
single node out there, so software can use it without being told what
addresses are available".  Not manually administered, not routed.

On the draft itself, *shrug*.  I dislike all the ways modern linux systems
use 127.0.0.<random> for magic things, but I don't have to use it, so I
could care less.

Gert Doering
        -- NetMaster
-- 
have you enabled IPv6 on something today...?

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