[CLUE-Talk] Are MAC addresses really unique?]

[Dave Price]

Interesting thought.

I think the key is that so many devices are flashable now.  I was even
told by AT&T broadband to reprogram a cable modem to a totally bogus
addres, so that their DHCP server would hand out a new IP address.

aloha,
dave

_______________________________________________________________

Today's focus:  Are MAC addresses really unique? 

By Steve Blass

Are media access control addresses really unique? Or are there 
any (maybe cloned?) network interface cards that have the same 
MAC address as another NIC? What is the probability of having 
two identical MAC addresses within one network?

The IEEE manages MAC addresses. The hardware identification 
addresses that the IEEE distributes are unique. That makes the 
probability of matching MAC addresses zero.

On the other hand, some hardware MAC addresses are programmable, 
which makes them spoofable. This means that it is possible for 
two machines in the same network to have the same MAC address.

To actually calculate the probability that two or more computers 
in the same network share the same MAC address, look up 
'selection with and without replacement' in an introductory 
Combinatorics or Probability and Statistics textbook and follow 
the procedures outlined in the book.

You can find network standards information available online at 
http://standards.ieee.org/. In addition to the FAQ files there, 
the IEEE recently has made the collection of 802 Ethernet 
networking standards documents available free of charge in PDF 
format through the Get IEEE 802 
<http://standards.ieee.org/getieee802/> program.

RELATED EDITORIAL LINKS

LAN research center
Latest news, analysis and research links.
http://www.nwfusion.com/topics/lans.html
_______________________________________________________________
To contact: Steve Blass

Blass is a network architect at Change at Work in Houston. He can 
be reached at mailto:dr.internet at changeatwork.com .
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This message was sent to:  davep at kinaole.org

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