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As far as I know, every machine has a globally unique MAC address. So why do we need local MAC address? We can identify the machines in a network using the globally unique MAC address.

  • "A locally administered MAC address is similar to a LAN IP address (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16). You can make up your own locally administered address and can be sure that it will not collide with any hardware on your network that use a factory burned-in MAC address." -- From noah.org/wiki/MAC_address – Rick Jul 14 at 0:08
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Not every device has a globally unique MAC address, and I'm not sure why you think that. Some layer-2 protocols (in particular the IEEE LAN protocols) use MAC addressing, but some use other addressing or no addressing at all. Of those that use MAC addressing, some use 48-bit MAC addresses, and some use 64-bit MAC addresses, but all are LAN (layer-2) protocols.

A vendor can buy an OUI from the IEEE, and they can assign the MAC addresses in its OUI as it sees fit. A vendor that makes ethernet, token ring, Wi-Fi, etc. NICs can reuse the same MAC address on all those. Some vendors reuse MAC addresses in different regions of the world.

If you mean the reason for the U/L bit in the OUI, that was part of the original specification. End-users can set that bit and assign MAC addresses in a way that works for them.

  • What is the "local MAC address" the question asked about? Is there such a category of "local mac address" and "globally unique mac address"? I've never heard that. – Rick Jul 13 at 23:25
  • Locally assigned MAC addresses have (or are supposed to have) the U/L bit set. The IEEE will assign a vendor an OUI, but the U/L bit is clear in those, and the vendor can assign any addressing within that OUI as it sees fit. End-users can change the MAC address on an interface, and are supposed to set the U/L bit when doing that to show that the address is locally assigned, Unfortunately, most people changing a MAC address do not follow the rules. – Ron Maupin Jul 13 at 23:32
  • Still don't get the point. So does the word "local MAC address" stands for those MAC addresses which are changed by end users? – Rick Jul 13 at 23:41
  • Well, it could mean different things depending on the context. For example, a MAC address is only relevant or seen on its broadcast domain, so it could be called local to the broadcast domain. It could also meas a locally assigned MAC address. You need to be more specific. – Ron Maupin Jul 14 at 0:02
  • Ah, I found the answer from Wikipedia and Google. "A locally administered MAC address is similar to a LAN IP address (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16). You can make up your own locally administered address and can be sure that it will not collide with any hardware on your network that use a factory burned-in MAC address." -- From noah.org/wiki/MAC_address . I think that's the reason. – Rick Jul 14 at 0:07

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