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

Thu Mar 6 04:51:14 MST 2003

Actually if you work in a very large PC shop you'll see duplicates.
They're rare and it's very uncommon to get multiples but I've seen a
couple duplicates at the same company before when we bought a LOT of
NIC's from one vendor.

A common MAC looks like this:

00:a0:cc:5d:78:15:00:30:80:ac:84:a8:08:00
which can also be represented as:
00a0:cc5d:7815:0030:80ac:84a8:0800

Depending on what your looking at (iptables reports the top version).
The regulating bodies give the vendor's the first half of the
addr:
00a0:cc5d:7815

which leaves the vendors the last 4 numbers for thier serial.  That's
alot of permeutations, but it is possible to get a dupe.

Robert

Thus spake Dave Price (davep at kinaole.org):

> 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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:wq!
---------------------------------------------------------------------------
Robert L. Harris                     | PGP Key ID: E344DA3B
                                         @ x-hkp://pgp.mit.edu 
DISCLAIMER:
      These are MY OPINIONS ALONE.  I speak for no-one else.

Diagnosis: witzelsucht  	

IPv6 = robert at ipv6.rdlg.net	http://ipv6.rdlg.net
IPv4 = robert at mail.rdlg.net	http://www.rdlg.net
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