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.
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"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– RickJul 14, 2019 at 0:08
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1 Answer
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.
answered Jul 28, 2017 at 13:50
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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.– RickJul 13, 2019 at 23:25
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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, 2019 at 23:32
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Still don't get the point. So does the word "local MAC address" stands for those MAC addresses which are changed by end users?– RickJul 13, 2019 at 23:41
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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, 2019 at 0:02
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1Well, I first had to disabuse the OP of his incorrect understanding of, "As far as I know, every machine has a globally unique MAC address." Burned-in MAC address assignments are completely handled by the vendor, who can assign them as it wishes. What is unique is the OUI assigned by the IEEE, not the MAC addresses themselves.– Ron Maupin ♦Jul 14, 2019 at 0:20