OMAPI Host Object and Duplicate MACs in different subnets
Simon Hobson
dhcp1 at thehobsons.co.uk
Wed Jul 9 06:48:45 UTC 2008
At 17:38 +0200 8/7/08, Stéphane PURNELLE wrote:
>It's possible to have multiple same MAC address ?
Officially no, in practice yes.
Every MAC address is supposed to be unique -
manufacturers are supposed to obtain one (or more
depending on quantity) 3 byte prefix from the
registry, and are then supposed to track which
combination of the last 3 bytes they have used.
In practice there are a number of problems :
'cheap' manufacturers who don't (or didn't) grasp
the importance of this and so just made up some
numbers
'not quite so cheap' manufacturers who bothered
to get a prefix, but then didn't track what
they've used
'cock ups' that can happen to even the biggest of
manufacturers. I heard that for a while, Dell
were duplicating one value in every 257 machines
shipped (ie of the 256 values for the last byte,
one got used twice).
'helpful' users or admins that reprogram a card
to match what they just took out
In small networks it doesn't matter as the
chances of getting a duplicate are slim. But as
the network gets bigger, the chances of having a
problem go up. Some (like the Dell problem) only
show up if you buy a lot of machines at the same
time - like a local university with a large flat
network and did an update by buying thousands of
machines in one order (I know one of the admins
and they were 'a bit puzzled' before they figured
out the problem !)
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