1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Duplicate MAC addresses?

Discussion in 'Networks' started by Baba O'Riley, Apr 2, 2006.

  1. Baba O'Riley

    Baba O'Riley Gigabyte Poster

    1,760
    23
    99
    Hey peeps,

    I was just reading my N+ book and it mentioned the (albeit rare) possibility of two devices being made with identical MAC addresses and was just wondering if anyone has come across this IRL and how it's resolved? The book mentions being able to alter the MAC address with software and implies you can do this with all NICs but I'm pretty sure you can't do it with mine, and what about other network devices with MAC addresses? I know I can clone my NICs MAC address on my router but that's about it.
     
    Certifications: A+, Network+
    WIP: 70-270
  2. hbroomhall

    hbroomhall Petabyte Poster Gold Member

    6,623
    115
    224
    I've never come across a duplicated MAC address. :biggrin

    It seems very variable whether the address can be changed - it is common on some broadband router/modems as a number of ISPs used that for authenticating, but fewer seem to do that now.

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  3. Baba O'Riley

    Baba O'Riley Gigabyte Poster

    1,760
    23
    99
    Well Harry, If you've never come across one, it's pretty safe to assume no one has :D .

    Thanks.
     
    Certifications: A+, Network+
    WIP: 70-270
  4. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

    6,199
    125
    199
    I don't know if MAC addresses can be changed as they're burnt onto the card. In theory though there may be an ability to 'flash' the card in much the same way that you do to a BIOS. But then it would depend on how the address was stored on the card in the first place.

    On a side note, when you team network cards up a 'fake' MAC address gets generated for the cards in the team, what is to stop this from being duplicated?

    :blink
     
  5. hbroomhall

    hbroomhall Petabyte Poster Gold Member

    6,623
    115
    224
    A dangerous assumption! I've heard of the problem, and a Google search shows that both Cisco and Intel have made such a booboo in the past.

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  6. hbroomhall

    hbroomhall Petabyte Poster Gold Member

    6,623
    115
    224
    That's a new one on me! Didn't know that could be done. I'll be Googling for a bit.....

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  7. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

    6,199
    125
    199
  8. Kestrel

    Kestrel Bit Poster

    17
    0
    14
    A MAC address, Media Access Carrier, is a 12-digit hexadecimal numbers (48 bits in length). It is burnt into the NIC and is not changeable in modern NIC's.

    MAC address are not infinate in number and if I caould be bothered I could workout the possible number of address supported by 12-digit hexadecimal numbers.

    The point being that it is possible to have duplicate MAC address on two NIC's but the chance of this is very very small. NIC manufactures apply for a range of MAC address's from the IEEE when they decide to manufacturer a run of NIC's. It is possible that they will get old MAC address's (I'm not sure if all the possible MAC address's have been used yet), but if this is the case then the chance of that NIC still being used in an online machine are virtually zero as we all know how fast technolgy moves on these days and the chance that an NIC that is only 5 yrs old being used are small enough and the cards that might have duplicated MAC's could be anything upto 15yrs old.

    Sorry for the blurb, but in short, getting a duplicate MAC address is almost zero and no, you cant change an NIC's MAC address.

    PS Yes I do rememebr the days of clearing chips with ultra violet light ;)

    PPS I got bothered and multiplied 2^48 and came out with 281,474,976,710,656. So as you can see, the chance of getting 2 NIC's with the same MAC address are very VERY small ;)
     
    Certifications: ECDL lvl2
    WIP: Profes Diploma in Network Managment
  9. Baba O'Riley

    Baba O'Riley Gigabyte Poster

    1,760
    23
    99
    Yup, that's what the book implied.

    281474976710656 :tongue

    Well, the first 24 bits of a MAC address are unique to the manufacturer, that's 16777216 potential manufacturers each being able to produce the same number of devices. Assuming, if a manufacturer did reach their limit they could just be issued a new range of numbers I'd be surprised if we were anywhere near using them all up but there is a massive global market in pirated hardware (especially in SE Asia) and I seriously doubt these rogue manufacturers apply to the IEEE for legitimate MAC addresses.

    Media Access Control :blink ?

    See here
     
    Certifications: A+, Network+
    WIP: 70-270
  10. Kestrel

    Kestrel Bit Poster

    17
    0
    14
    Media Access Carrier / Media Access Control

    Redundant Array of Inexpensive Discs / Redundant Array of Independant Disks

    They are both the same, all depends on where you do your reseach.

    And as for Macshift, looking at that it looks like a 'virtual' change of the MAC address within the registry to me and not changing the 'physical' MAC address burnt onto the NIC it's self. Though who would want to change MAC address's on a NW i'm not sure about as that can all be done with IP addressing within a NW. I really think you'll find it is not possible to change the burnt in MAC address on a modern NW card.
     
    Certifications: ECDL lvl2
    WIP: Profes Diploma in Network Managment
  11. Baba O'Riley

    Baba O'Riley Gigabyte Poster

    1,760
    23
    99
    I'm happy to accept that, it wasn't what I asked (actually, if you look right at the bottom of the link I posted you'll see there's at least one manufacturer that puts the MAC address on EEPROM so it can be changed). I asked if anyone had come across duplicated MAC addresses which, however unlikely, you must admit is possible. As for the fix, there seems to be several options depending on OS etc.
     
    Certifications: A+, Network+
    WIP: 70-270
  12. Kestrel

    Kestrel Bit Poster

    17
    0
    14
    I'll accept its possible, but with over 281 trillion possible MAC address out there the chances are small and I still hold onto the notion, 'who would want to change their MAC address?'

    Sure its possible with EERPROM to change the MAC address, but when was the last time anyone saw one of those? We have one lying around in the lab at college, but there isnt a machine in the building it will fit onto.

    http://www.sdadapters.com/faq.htm

    After reading that I would go near it with a barge pole in all honesty. Designing and implementing a network is time consuming enough as it is. Changing MAC address is just going to complicate the thing even more.

    Its like buying a huge rucksack, remember that if you fill it up you have to carry it. Just the same if you impement a NW with all these fancy whislte and bells on it. You'd better make sure you document the procedure to the Nth degree, because if you dont, then in a couple of yrs when the NW has grown and you need to maintain it, without proper documentation, your gonna find it very hard to rememebr just what it is that you did. Changing MAC address is just going to complicate even more what is already not a straight forward operation.

    Forgert about MAC address's m8, consider them set in stone and worry about the other fundamentals of setting up a server such as OU GUID's and policies.

    Your time will be taken up enough with those considerations as it is without throwing another variable into the equation.
     
    Certifications: ECDL lvl2
    WIP: Profes Diploma in Network Managment
  13. Baba O'Riley

    Baba O'Riley Gigabyte Poster

    1,760
    23
    99
    Anyone worth their salt would document their network in detail anyway.


    I think you're making a bigger deal out of this than I intended. It's not like I'm never going to rest until I find an example of this. I came across the idea in the normal course of things and found it an interesting notion so I thought I'd put it out there while I was on a break. Trust me, I'm not going to lose any sleep. :)
     
    Certifications: A+, Network+
    WIP: 70-270
  14. Baba O'Riley

    Baba O'Riley Gigabyte Poster

    1,760
    23
    99
    I wasn't going to post in this thread anymore but I've been mulling on what you said:
    You and I are studying very different courses towards different qualifications. Now, for the purposes of your course they may well be set in stone, but while I don't think this matter is mentioned directly in the Network+ objectives, it was mentioned in the book for a reason, ie. that it's worth being aware of in case it does come up in an exam. If the A+ is anything to go by, then anything even remotely relevant is fair game for the exams so, if it's all the same to you, I won't forget about it and I won't consider them set in stone because, as has been demonstrated, they are not.
     
    Certifications: A+, Network+
    WIP: 70-270

Share This Page

Loading...