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

[Mark Smith <[e-mail]>]

Hi,

On Wed, 26 Nov 2025 at 20:06, Gert Doering <[e-mail]> wrote:
>
> Hi,
>
> On Wed, Nov 26, 2025 at 07:20:17PM +1100, Mark Smith wrote:
> > On Wed, 26 Nov 2025 at 18:26, Gert Doering <[e-mail]> wrote:
> > > On Wed, Nov 26, 2025 at 11:03:20AM +1100, Mark Smith wrote:
> > > > On Wed, 26 Nov 2025 at 10:19, Geoff Huston <[e-mail]> 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...).
> > >
> >
> > Yes. I mentioned it to demonstrate that I think a larger loopback
> > prefix should be big enough to support multiple loopback interfaces
> > within a host.
>
> I find this very hard to understand.
>
> Why would "there is a larger loopback prefix, used only for machine-internal
> communication" have any relevance to "I use multiple loopback interfaces with
> manually configured routable IP addresses"?
>
> Why would "having multiple loopback interfaces out of the well-known
> machine-internal loopback address space" have any relevance to anything?
>

It's in the context of the draft I wrote proposing a larger IPv6
prefix. The prefix should be large enough to support subnetting it so
that those sub-prefixes can be assigned to multiple loopback
interfaces on a host.

A ::/96 prefix means having to subnet in an unfamiliar location within
the IPv6 address.

A /48 for the loopback prefix would allow subnetting at the familiar
/48 boundary, resulting in familiar /64s sized subnets that are
assigned to different loopback interfaces on the host.

Assignment of loopback prefixes to loopback interfaces should be
automated. I think that is one of the key values of 127.0.0.1/8 - it's
automatically configured. Using at least /48 and then a /64 per
loopback interface allows doing something like using the interface's
ifIndex as the /64 subnet number (ifindex are typical 16 bits in size,
same as the /48 through /64 bits).

Loopback interfaces are required to have a link-local prefix per
rfc4291, and their prefix length is a /64. The drawback of using a
link-local address for testing over a loopback interface is that they
require a zone identifier.

While a /48 would be fine, since it's better to try to avoid having to
grow the loopback space in the future (e.g. which is what has happened
with the documentation prefix), a /32 or 1 x 4 billionth of the IPv6
address space, for a larger loopback prefix seems worth while.

Regards,
Mark.

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