Real world use for the U/L bit?

Brian E Carpenter brian.e.carpenter at
Mon Nov 15 01:02:54 CET 2010


You can certainly do that (i.e. static addressing) and if you do that,
I suppose you should choose a static address with u/l set to zero,
indicating 'local'.

Thus 2001:4860:b006::63, which sure looks static to me.

   Brian Carpenter

On 2010-11-15 12:24, bmanning at wrote:
>  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

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