Re: [LARTC] multiple MAC addresses from one network card

[Breen Mullins <bmullins@pacbell.net>]

On Fri, 6 Apr 2001, James A. Crippen wrote:

> I have very occasionally encountered two cards from a manufacturer with
> the same address.  This has happened only once or twice in my life, but it
> does happen, mostly because there are only so many bits for the
> manufacturer to play with, usually only 32 or 24 of the whole 48 bit
> address. 

It's 24 bits for the manufacturer to play with. The first three octets are
assigned by IEEE and the manufacturer assigns the other three.

Duplications happen sometimes, mostly because of a manufacturing error,
but occasionally because the manufacturer neglected to order a new address
range in time from IEEE.

If you need to assign extra MACs to a card, you should set the "locally
assigned" bit. It's the second bit on the wire and in canonical form
that's the 0x02 bit of the first octet, so if the  real MAC is of the form 
00:00:0c:xx:xx:xx you could assign 02:00:0c:xx:xx:xx with no fear of a
collision.

That's the theory, anyway. I've never had occasion to try it.

Breen


-- 
Breen Mullins
San Mateo, Calif.
<mailto:bmullins@pacbell.net>



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