<div dir="ltr"><div class="gmail_default" style="font-family:arial,sans-serif;color:rgb(0,0,0)">Yeah, some of the RFCs on v6 address formats hem and haw about how big the network ID and interface ID parts are (probably written before actual implementations were in place), but <a href="https://www.rfc-editor.org/rfc/rfc4291#section-2.5.1">https://www.rfc-editor.org/rfc/rfc4291#section-2.5.1</a> says quite unequivocally:<br><pre class="gmail-newpage"> For all unicast addresses, except those that start with the binary
value 000, Interface IDs are required to be 64 bits long...<br><br></pre><pre class="gmail-newpage"><span style="font-family:arial,sans-serif">Which drives a stake in the ground regarding how to partition those 128 bits.</span></pre>
</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Jun 20, 2023 at 11:59 AM Petr Menšík <<a href="mailto:pemensik@redhat.com">pemensik@redhat.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<p>I think that is required by SLAAC RFC, which adds another 2 bytes
to 6 bytes of hardware ethernet address.</p>
<p>Which is in total 8 bytes, therefore 64 bits is required for it.
Prefix cannot be higher, but can be lower in theory. There might
be some implementation details now supporting lower prefix length
in current implementation.</p>
<p>Cheers,<br>
Petr<br>
</p>
<div>On 15. 06. 23 12:07, renmingshuai via
Dnsmasq-discuss wrote:<br>
</div>
<blockquote type="cite">
<div>
<p class="MsoNormal" style="line-height:130%"><span lang="EN-US">When
ra-only, slaac, or ra-stateless is configured in dhcp-range
and the prefix len is set to a value other than 64, like
this:<u></u><u></u></span></p>
<p class="MsoNormal" style="line-height:130%"><span lang="EN-US">“dhcp-range=2000:1000:1000:1000:1000:1000::,
ra-stateless,120,infinite”<u></u><u></u></span></p>
<p class="MsoNormal" style="line-height:130%"><span lang="EN-US">the
following error message is displayed:<u></u><u></u></span></p>
<p class="MsoNormal" style="line-height:130%"><span lang="EN-US">dnsmasq:
prefix length must be exactly 64 for RA subnets at line 16
of /etc/dnsmasq.conf<u></u><u></u></span></p>
<p class="MsoNormal" style="line-height:130%"><span lang="EN-US">Why
must the prefix length be 64? This may come from an RFC
regulation or recommendation, but I didn't find it. Would
you mind tell me the reason?<u></u><u></u></span></p>
</div>
</blockquote>
<pre cols="72">--
Petr Menšík
Software Engineer, RHEL
Red Hat, <a href="http://www.redhat.com/" target="_blank">http://www.redhat.com/</a>
PGP: DFCF908DB7C87E8E529925BC4931CA5B6C9FC5CB</pre>
</div>
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