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Proper MAC Adrress format ?

Discussion in 'General Discussion' started by Nicole, Mar 29, 2009.

  1. Nicole

    Nicole Addicted to LI Member

    Does it matter what the last 2 digits of a MAC address are?
  2. fyellin

    fyellin LI Guru Member

    All bits of a MAC address are significant. And there are not any specifically reserved values that I know of.

    You may be confused by the fact that your router has (at least) three MAC addresses, one for the LAN, one for the WAN, and one for the Wireless. These three MAC addresses are in sequence on some routers.
  3. Nicole

    Nicole Addicted to LI Member

    I'd like to change my router's MAC address, so I could request a new IP from my ISP.

    If I enter an invalid MAC, will my router stop working?
  4. fyellin

    fyellin LI Guru Member

    There are various restrictions on the high byte, but as long as you leave it zero, you're safe. The only other sort-of restriction is that you don't want to have the same MAC as some one else on your ISP's network. But if you use a random address, your odds of collision are small.

    If you want an address that you know is valid, use the LAN address of some device on your network as the WAN address.
  5. fyellin

    fyellin LI Guru Member

    I just checked out the code on Tomato for generating a random MAC address. It sets the first byte to 0, and all subsequent bytes to a random value between 0 and 254, inclusive. It avoids setting any byte to 255 (0xFF), though I don't know if this is accidental or intentional.

    In any case, you won't break your router.
  6. Nicole

    Nicole Addicted to LI Member

    I'm running DD-WRT, Tomato doesn't work on my router.

    I'm keeping the entire MAC Address the same except for the last 2 characters:
    For the last two characters I am doing random letter [A through F] + random number [0-9]
  7. fyellin

    fyellin LI Guru Member

    Sorry, you misread my intentions. I wasn't trying to convert you to Tomato. Just to let you know that since Tomato does this, it's almost certainly safe.

    You should have no problem.

    Do remember that with some cable companies, you need to turn the cable modem off and then back before it'll give you a different IP.
  8. Nicole

    Nicole Addicted to LI Member

    Thanks for your help fyellin :)

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