OMAPI Host Object and Duplicate MACs in different subnets

[Simon Hobson]

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 !)