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

[Ole Trøan <[e-mail]>]

Mark,

Since the loopback address is of link-local scope you can already do this with multiple loopback interfaces. 
::1%loop0
::1%loop1

Cheers,
Ole

> On 26 Nov 2025, at 11:34, Mark Smith <[e-mail]> wrote:
> 
> 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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> 
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