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From: geoff@col...
Subject: unique MAC address allocation?
Date: Wed, 30 Mar 2005 03:20:37 -0800

Manufacturers can request (or perhaps buy) a largish block of
addresses from some standards organisation (I think it's
Bellcore/Telcordia/whatever-they're-called-this-week).  If they
exhaust that block, they can get another, and so on.

Manufacturers are supposed to ensure uniqueness of MAC addresses, but
they've been known to screw up (e.g., SGI), so yes, two NICs are
supposed to have different MAC addresses.  In practice, since we don't
run XNS, a MAC address only has to be unique within the Ethernet that
its card is plugged into (assuming no BOOTP through gateways).

Many cards, not just new ones, permit overriding the default MAC
address and the plan 9 drivers generally support this (with
ea=XXXXXXXXXXXX in the ether's plan9.ini entry).