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I am trying to designate a MAC address for MS loopback adapter. Installation is fine. I see in device manager, the adapter and in advanced tab, I enter the mac address I desire. But when I do in cmd prompt ipconfig -all, I see the original mac address is retained.

I did search a bit but google is not extremely useful. I am hoping an easy solution exists to this problem. My os is XP.

thanks, KD

4
  • why would you need to change the MAC of the loopback adapter?
    – Riguez
    Apr 21, 2011 at 5:02
  • 1
    Since there is no medium, there is no MAC address really. Why do you want to change it? Even if windows is silly enough to list a bogus address, you gain nothing by changing it.
    – psusi
    Apr 21, 2011 at 13:46
  • Well, I am googling this in 2017 because it may be the only way to move a (legitimately purchased!) node-locked software license onto a new computer, as some programs use a MAC address for licensing. Not sure if it'll work for my use case (probably not because of the locally-administered restriction), but this question has been helpful.
    – Dmitri
    Apr 19, 2017 at 20:38
  • Apparently someone else had the same question too: eightforums.com/network-sharing/…
    – Dmitri
    Apr 19, 2017 at 20:41

2 Answers 2

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It seems that the adapter will accept only locally administered MAC addresses as valid (makes sense). This is signified by 7th leftmost bit set to 1, so MAC must start with 02. I changed it without any problems: enter image description here

Ethernet adapter Local Area Connection 3:

    Connection-specific DNS Suffix  . :
    Description . . . . . . . . . . . : Microsoft Loopback Adapter
    Physical Address. . . . . . . . . : 02-FA-FA-FA-FA-FA
    Dhcp Enabled. . . . . . . . . . . : No
    IP Address. . . . . . . . . . . . : 10.0.0.1
    Subnet Mask . . . . . . . . . . . : 255.255.255.0
    Default Gateway . . . . . . . . . :

If you try to define it as Globally unique (00), it treats it as invalid and reverts to default: enter image description here

Ethernet adapter Local Area Connection 3:

    Connection-specific DNS Suffix  . :
    Description . . . . . . . . . . . : Microsoft Loopback Adapter
    Physical Address. . . . . . . . . : 02-00-4C-4F-4F-50
    Dhcp Enabled. . . . . . . . . . . : No
    IP Address. . . . . . . . . . . . : 10.0.0.1
    Subnet Mask . . . . . . . . . . . : 255.255.255.0
    Default Gateway . . . . . . . . . :
0

The custom MAC address may need to conform to the following format:

  • x2-xx-xx-xx-xx-xx-xx
  • x6-xx-xx-xx-xx-xx-xx
  • xA-xx-xx-xx-xx-xx-xx
  • xE-xx-xx-xx-xx-xx-xx

x can be any number from 0 to F.


The reason is as follows:

  1. MAC address - I/G bit

    The least significant bit of an MAC address's first octet is referred to as the I/G (Individual/Group) bit.

    • 0: Unicast MAC Address
    • 1: Multicast MAC Address

    The I/G bit of a custom mac address needs to be set to 0.

  2. MAC address - U/L bit

    the second-least-significant bit of an MAC address's first octet is referred to as the U/L (Universal/Local) bit.

    • 0: Universally Administered MAC Address
    • 1: Locally Administered MAC Address

    The U/L bit of a custom mac address needs to be set to 1.

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