IPV6 planning

Saku Ytti saku at ytti.fi
Sun Mar 6 11:53:27 UTC 2016

On 6 March 2016 at 03:08, Karl Auer <kauer at biplane.com.au> wrote:
> To support SLAAC with prefix lengths other than 64 you would have to
> break numerous standards. RFC2464 is very clear on the matter, at least
> for Ethernet interfaces, though RFC 4862 is carefully non-committal.

Yes, SLAAC, 4862 clearly does not forbid it, and there is no technical reason.
But as you state, 2464 does not specify other behaviour. Writing new
draft which specifies behaviour for arbitrary size wouldn't be a
challenge, marketing it might be.

>From implementation POV, arbitrary prefix-size from 4862 is fairly
obvious and logical, and dictating 64 is very weird, with no obvious
benefits at all. I suspect the 64 must have come into IPv6 ethernet
standard before people thought of DAD, and assumption was uniqueness
was guaranteed by use of BIA (HAH!), that's only background that I can
imagine where mandating /64 might be remotely sane thought process.
Post-DAD it's just artificial complexity which reduces expressiveness.

> Even if the router supports it, as far as I know a standards-conforming
> host MUST ignore such prefixes for purposes of SLAAC on Ethernet.

I can't recall where it would be worded so harshly. But I agree that
it's not good idea /NOW/ to use it, even if it works on some


More information about the NANOG mailing list