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

[Owen DeLong <[e-mail]>]

I think that use case is better served using GUA or ULA on loopback interfaces as these addresses really aren’t performing a loopback address function. 

For the purpose of loopback addresses, I don’t see /64 or larger as being worth the conflict with ::ffff:0:0/96. 

Owen


> On Nov 25, 2025, at 18:18, Mark Smith <[e-mail]> wrote:
> 
> Hi Geoff, all,
> 
>> On Wed, 26 Nov 2025 at 11:48, Geoff Huston <[e-mail]> wrote:
>> 
>> Hi Mark,
>> 
>> I hear you, but I also observe that a prior effort to expand this designation
>> in 2013 did not get beyoind a draft. This incarnation of the proposal is deliberately
>> more modest (even then I see David Farmer saying (paraphrased) "whoa! Way
>> too much!" So we have one comment saying "too big" and another saying "too
>> small"!
>> 
> 
> I would argue for what is a convenient larger loopback prefix size to
> suit all reasonable and conceivable use cases, not just what is
> minimal and necessary. That's the freedom we have in IPv6 that we
> haven't had in IPv4 since RFC791 when classes were introduced (vs
> RFC760s 8 bit network numbers and 24 bit host numbers).
> 
>> 
>> I'm inclined to say that the draft will keep this proposal  at a /96 for now but
>> doubtless Warren and I will keep a close eye on any other comments
>> that have a view about the appropriate size for a loopback prefix.
>> 
> 
> I think using a /96 for a loopback interface would be shoehorning IPv4
> addressing into IPv6 rather than following IPv6's addresses
> conventions. At a minimum I think it should be a /64 to support 64 bit
> IIDs on the loopback interface that can be automatically generated by
> the host at boot time via RFC7217 and/or RFC8981 IIDs.
> 
>> But I do note:  "Those loopback interface addresses were all announced into
>> the routing protocol" might bne interpreted as a bit contrary to RFC 4291
>> which says quite  definitively "keep loopback em to yourself" So I am not
>> exactly on board with a rationale that relates to a use scenario in routing
>> protocol that "exports" these addresses out to neighbours, if I interpret your
>> description correctly.
> 
> I probably confused things. I should have said "loopback *interface*"
> addresses. They weren't from 127/8, they were standard unicast IPv4
> addresses. Using loopback virtual interfaces and announcing those /32s
> into the routing protocol was for the purposes of making them
> reachable via any of the router's physical interfaces for BGP sessions
> etc.
> 
> I was using that example to demonstrate the value of supporting
> multiple loopback interfaces on a node/host, and in conjunction with
> supporting IPv6 addressing conventions, that the larger loopback
> prefix should be large enough to provide multiple /64s at a minimum. A
> /48 would suffice, however a /32 is cheap enough so that's why I
> bumped the size up from a /48 to a /32 in my larger loopback ID.
> 
> Regards,
> Mark.
> 
> 
> 
>> 
>> regards,
>> 
>>  Geoff
>> 
>> 
> 
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