Real world use for the U/L bit?

Roger Wiklund roger.wiklund at
Sun Nov 14 11:26:27 CET 2010


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

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.



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