Real world use for the U/L bit?

Brian E Carpenter brian.e.carpenter at
Sun Nov 14 21:38:23 CET 2010


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.

   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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