CC: s to Non List Members (was Re: 202203080924.AYC Re: 202203071610.AYC Re: Making Use of 240/4 NetBlock)

John Levine johnl at
Wed Mar 9 00:14:34 UTC 2022

It appears that William Herrin <bill at> said:
>On Tue, Mar 8, 2022 at 12:34 PM John Levine <johnl at> wrote:
>> FWIW, I also don't think that repurposing 240/4 is a good idea.  To be useful it would require
>> that every host on the Internet update its network stack,
>Hi John,
>That's incorrect and obviously so. While repurposing 240/4 as general
>purpose Internet addresses might require that level of effort, other
>uses such as local LAN addressing would only require the equipment on
>that one lan to be updated -- a much more attainable goal.

If you want to patch your devices so they use 240/4 as a version of
10/8 on your own network, you can do that any time you want.

>Reallocating 240/4 as unpurposed unicast address space would allow
>some standards-compliant uses to become practical before others. A few
>quite quickly.

So long as we agree that "quickly" means a decade.  If we did this bad idea,
at some point there would be a tipping point where enough hosts recognized
them to be useful, but we say the same thing about IPv6.

>Is it not past time we admit that we have no real idea what the
>schedule or level of effort will be for making IPv6 ubiquitous?

Oh, absolutely.  I have conversations with my hosting provider in which
they tell me that nobody has ever asked for IPv6 other than me, and they
had no idea their upstream (Spectrum) had native IPv6.  So I keep using
a tunnel.  I would expect the same conversations about 240/4.


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