Real world use for the U/L bit?

[George Bonser]

Oops, sorry, I was thinking about the bit in the local addressing that
is "reserved".

My apologies.



> -----Original Message-----
> From: ipv6-ops-bounces+gbonser=seven.com at lists.cluenet.de
[mailto:ipv6-
> ops-bounces+gbonser=seven.com at lists.cluenet.de] On Behalf Of
> bmanning at vacation.karoshi.com
> Sent: Sunday, November 14, 2010 3:25 PM
> To: Brian E Carpenter
> Cc: Roger Wiklund; ipv6-ops at lists.cluenet.de
> Subject: Re: Real world use for the U/L bit?
> 
> 
>  and I thought it was for defining server addresses that were not
> dependent
> on a given MAC address, like DNS service, You don't
> want to keep changing the IP address just becuase the MAC has changed.
> Same is true for any BGP speaker.
> 
> --bill
> 
> 
> On Mon, Nov 15, 2010 at 09:38:23AM +1300, Brian E Carpenter wrote:
> > Roger,
> >
> > Today - it is no use. Just a rule.
> >
> > The reason it is in the address architecture is stated explicitly
> > in RFC 4291:
> >
> >   "The use of the universal/local bit in the Modified EUI-64 format
> >    identifier is to allow development of future technology that can
> take
> >    advantage of interface identifiers with universal scope."
> >
> > This was in the hope of a multihoming solution that would avoid PI
> prefixes.
> > That is still an active hope.
> >
> > Regards
> >    Brian Carpenter
> >
> >
> > On 2010-11-14 23:26, Roger Wiklund wrote:
> > > Hi
> > >
> > > I'm having problems understanding the _use_ for the U/L bit.
> > >
> > > I understand the concept, but I don't understand the use for it.
> > >
> > > The U/L bit is the seventh bit of the first byte and is used to
> > > determine whether the address is universally or locally
> administered.
> > > If the U/L bit is set to 0, the IEEE, through the designation of a
> > > unique company ID, has administered the address. If the U/L bit is
> set
> > > to 1, the address is locally administered. The network
> administrator
> > > has overridden the manufactured address and specified a different
> > > address.
> > >
> > > So if the EUI-64 address is created using the OUI, its universally
> > > administered. If the MAC is manually configured for some reason,
> its
> > > locally administered.
> > >
> > > But why do I need to know this? When will I actually have to login
> to
> > > a router/whatever to check how the U/L is set? We still have DAD
if
> > > two admins accidently use the same manually configured MAC address
> or
> > > something stupid like that.
> > >
> > > Thanks!
> > >
> > > /Roger
> > >