Class D addresses? was: Redploying most of 127/8 as unicast public

Greg Skinner gregskinner0 at
Mon Nov 29 18:47:14 UTC 2021

> On Nov 24, 2021, at 5:08 PM, William Herrin <bill at> wrote:
> On Wed, Nov 24, 2021 at 4:36 PM David Conrad <drc at> wrote:
>>> I like research but what would the RIRs study? The percentage of the
>> Lots of people said similar things when was allocated to APNIC
>> and they said similar things when was stood up as an
>> experiment by Cloudflare and APNIC, yet seems to be pretty popular.
> Hi David,
> I don't recall there being any equipment or software compatibility
> concerns with If you do, feel free to refresh my memory. As
> I recall it, there were concerns with prior local use and potential
> trash traffic. It seemed likely those concerns could be addressed with
> experiments, and the experiments in fact addressed them.
> The prior local use worry reared its head again with 240/4 but given
> the prior experience with I don't personally believe we need
> to re-run that experiment just because the numbers are part of a
> different block.
>> Seems to me that a number of folks on this list and during this
>> discussion would disagree with a blanket assertion that 240/4
>> is “dysfunctional on the 2021 Internet” - some of them even
>> wrote a draft discussing the possibility.
> Line them up. Show of hands. Who really thinks that if we assign
> to a customer tomorrow without waiting for anyone to clean
> up their software and hardware, you won't get enough complaints about
> things not working that you have to take it back and assign a
> different address instead?
>> 1. Move 240/4 from "reserved" to "unallocated unicast"
>> OK, but this seems like a quibble.  The status for 240/4 is “
>> RESERVED: designated by the IETF for specific non-global-unicast
>> purposes as noted.”  The “as noted” part is “Future Use”.
> It's not a quibble. Some vendors take the current status to mean
> "treat it like unicast until we're told otherwise." Others take the
> status to mean, "packets with these addresses are bogons without a
> defined routing behavior until we're told otherwise." The result is
> incompatible behavior between vendors. Changing that direction to
> "treat it like unicast" without ambiguity is not a quibble.
> Regards,
> Bill Herrin
> --
> William Herrin
> bill at

For what it’s worth, I’ve been tracking this issue on other netops mailing lists.  There is a recent post on the LACNOG list from Leandro Bertholdo <> referencing <>, a draft proposing another way to make additional IPv4 address space available.  I haven’t had time to read the draft closely, but I noticed that it involves the use of 240/4.  Subsequent googling about the draft turned up a presentation <> describing how the techniques described could be deployed.  I noticed that the presentation made reference to OpenWRT, so perhaps the authors are aware of the work that the authors of the IPv4 Unicast Extensions Project have done in that area.

The adaptive-ipv4 draft will expire sometime next month, so anyone interested in seeing it move forward should contact the authors.


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