> This is why they have a separate EEPROM on the board. My understanding is
the
> ASIC directly reads from the EEPROM as part of its initialization process,
not
> the driver transferring it to the ASIC. The driver can then access the
MAC, and
> presumably change it if desired.

There are lots of ways of doing this.  I worked on an ethernet switch once,
and the MAC address was taken from one of those Dallas Semiconductor little
non-volatile RAMs with the one-wire serial interface.  These can be (as can
be other PROMs) bought serialized within specific ranges of numbers.

Keep in mind that ethernet MAC addresses are not arbitrary.  Every ethernet
product in the world is initialized to a unique MAC address, and address
ranges are assigned to different manufacturers by the IEEE.  You can't just
go changing the MAC address unless you know exactly what you are doing.

> Whichever way it occurs, I still do not recall anybody mentioning what
happens
> if you have multiple copies of the same MAC address on the same network.
My
> memories of watching a network sniffer was that everything was related to
the
> MAC address in terms of where the source and destination of the message
were
> concerned. It is now a long time ago since I did this, so the memory may
be
> getting hazy on this.

Individual ethernet packets are addressed to MACs at the lowest level.
Higher level protocol may deal with sockets, IP addresses and the like.  The
ethernet hardware knows nothing about this and deals with MAC addresses
only.  Two devices on the same segment with the same MAC address will cause
major trouble.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@cognivis.com, http://www.cognivis.com

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