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

[David Farmer <[e-mail]>]

Mark, the problem is that ::0/48 includes ::ffff:0:0/96, which is allocated
for IPv4-Mapped IPv6 Addresses.

Thanks

On Tue, Nov 25, 2025 at 6:03 PM Mark Smith <[e-mail]> wrote:

> Hi Geoff,
>
> 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).
>
> I thought to do that so that if for some reason you might need to move
> the L2TP end point to a different router you could do that without
> disrupting your BGP sessions or management address for the router etc.
> A dedicated address per function made moving that function/service
> address to another router much easier than having to renumber other
> devices to point to a new location for the service/function. You could
> also configure different per interface packet filters for each of the
> loopback interfaces based on their security needs.
>
> >From that sort of experience I realised that having multiple loopback
> interfaces on a host could also be useful for testing etc., because
> you're able to simulate both the interface and address per interface
> model.
>
>  (Under Linux you can create additional "loopback" interfaces with the
> dummy module e.g. modprobe dummy numdummies=16 to create 16 dummy
> "loopback" interfaces).
>
> A /48 for the loopback prefix would do the trick, you could allocate a
> /64 to each of the loopback interfaces. However, for the purposes of
> routing testing and simulation I thought possibly you might want to
> have each loopback interface represent a subscriber CPE to which you
> assign a /48. So a /32 seemed to me to be a better "Goldilocks" size
> for the loopback prefix that would best and better accommodate all
> likely testing scenarios on a host that you would use a loopback
> prefix for, and it also aligned with the default RIR prefix assignment
> size.
>
> Regards,
> Mark.
>
>
> > :-)
> >
> > g
> >
> >
> > > On 26 Nov 2025, at 10:16 am, Mark Smith <[e-mail]>
> wrote:
> > >
> > > Hello,
> > >
> > > On Wed, 26 Nov 2025 at 04:40, Warren Kumari <[e-mail]> wrote:
> > >>
> > >> Dear 6MAN and V6OPS,
> > >>
> > >> Geoff Huston and I have just submitted draft-kumari-ipv6-loopback -
> "The IPv6 Loopback Address Prefix"
> > >>
> > >> We believe that it is within the 6MAN charter ("The 6man working
> group is responsible for the maintenance, upkeep, and advancement of the
> IPv6 protocol specifications and addressing architecture."), but I have
> CCed V6OPS as well, as it is clearly operational as well.
> > >>
> > >> Abstract:
> > >> "This document updates the IP Version 6 Address Architecture to
> define the IPv6 address prefix ::/32 as the Loopback address prefix."
> > >>
> > >> Basically, this document expands the single loopback address ::1/128
> into a prefix.
> > >>
> > >> Yes, we are aware that there have been some previous discussions[0]
> on the need (or lack thereof!) of a loopback prefix in IPv6, but we believe
> that they are worth revisiting.
> > >>
> > >> There are a number of situations in which having more than a single
> address is helpful; an obvious example of this is Dockers/k8s use of
> 127.0.0.11 for the DNS resolver, SPAM RBL use of the last octet on
> 127.0.0.x to encode the type of SPAM. It is also relatively common it use
> this for inter-service communication in container environments.
> > >>
> > >> It is also a common practice to bind different services to different
> addresses in the IPv4 loopback space to allow for scaling (avoiding the
> "Port already in use" issue), testing, etc.  Yes, these can be somewhat
> emulated with ULAs and / or additional interfaces and scopes, but they are
> all more complicated, and much more likely to result in leakage or
> collision.
> > >>
> > >> Another, more recent example is the ICANN Public Comment on "Name
> Collision IPv6 Research Study" and proposed use of ::ffff:7f00:3535 [1] -
> if there was a loopback prefix this would have been a better option[2]
> > >>
> > >
> > > I fully agree that there is a need for a larger loopback prefix. I
> > > also fully agree that /32 is the Goldilocks size.
> > >
> > > "A Larger Loopback Prefix for IPv6"
> > >
> https://datatracker.ietf.org/doc/draft-smith-v6ops-larger-ipv6-loopback-prefix/04/
> > >
> > > Regards,
> > > Mark.
> > >
> > >
> > >>
> > >> We expect a fairly robust discussion :-),
> > >> W
> > >>
> > >> [0]: I know I've seen them, but I quick search of my mail was unable
> to find these — the authors are more than happy to link to previous
> documents, etc.
> > >>
> > >> [1]: See long threads on 6MAN
> https://mailarchive.ietf.org/arch/msg/ipv6/-HrYFMwHhsUWYxSXsFIkLpF_Qgk/
> and V6OPS.
> > >>
> > >> [2]: Solving the technical concerns, but not necessarily the policy
> ones.
> > >>
> > >> _______________________________________________
> > >> v6ops mailing list -- [e-mail]
> > >> To unsubscribe send an email to [e-mail]
> >
>
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