IPv6 issue on TestBot?

Michael Stefaniuc mstefani at redhat.com
Thu Sep 22 09:54:23 CDT 2011

Per Johansson wrote:
> 22 sep 2011 kl. 12:14 skrev Michael Stefaniuc:
>> Wow! /66? And that works? While the standard allows for that you "should
>> use /64" which everybody and his dog read it as that is the only thing
>> that needs to work and the only thing that get tested. IPv6 brings back
>> the class-full thinking which everybody has to painfully unlearn once
>> IPv6 catches on...
>> "Safe" prefix length (especially if involving client devices) are:
>> /64 - LAN
>> /126 and /127 - point to point
>> /128 - host routes
> RFC3513 is quite more strict than "should":
Which is obsoleted by RFC4291, but that seems to have kept this
paragraph. Anyway the correct RFC term to make it mandatory is "must".
And it isn't a "must". Also the RFC4291 is from 2006 which is *old*.

>    All global unicast addresses other than those that start with binary
>    000 have a 64-bit interface ID field (i.e., n + m = 64), formatted as
>    described in section 2.5.1.  Global unicast addresses that start with
>    binary 000 have no such constraint on the size or structure of the
>    interface ID field.
> (2001:: has the binary prefix 001). So this might very well be the problem.
The same RFC4291 says:
   Except for the knowledge of the subnet boundary discussed in the
   previous paragraphs, nodes should not make any assumptions about the
   structure of an IPv6 address.

It might be the problem but not because of the RFC; it just might be a
typo as it is a manually set IP.

> I've heard the reason for 64 bits is that it's what currently fits in most computer registers, but I don't know if it's true.
What really helps to understand IPv6 is:
- IPv6 is an old protocol >15 years. That predates the Internet taking over.
- IPv6 was designed based on the assumption that it will make the
Internet again a friendly, warm and cozy place; a big any to any network
without barriers/firewalls.
- IPv6 isn't better than IPv4, it is different.
- IPv4 still outperforms IPv6 in the new feature development. IPv6 is
trying to play catch up. A few things that were long ago deprecated in
IPv4 are still mandatory for IPv6 to be "standards compliant".
- Don't assume that if the IPv6 standard says something the
implementations really follow that. Test, test, test.

So no, the /64 host part has nothing to do with current hosts having
64bit registers now. I'm pretty sure that they were hoping to have
128bit computers by now. It had more to do with thinking that a globally
unique identifier for the host is a good think to have and the MAC
address already provided that with 48bits. But 48 isn't a nice number so
it was rounded up to 64 which makes for a nice "design" as it is half of
the IPv6 address.


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