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If a Wi-Fi device is design so there will only ever be one on a network, is there any issue with randomly assigning the MAC address from a small pool? Globally there will be duplicates, but never locally.

It seems either a CID could be used as a prefix or a MA-S small block could be used to prevent conflicts with other devices on the network.

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    A "built-in" MAC must be globally unique. You have no idea, or control, over the network in which it will be placed. IEEE really frowns on OUI holders doing this sort of thing. If you're talking about run-time generated addresses, then avoiding collisions is your problem. (see also: dd-wrt, vmware, etc.)
    – Ricky
    Jul 19 '16 at 19:37
  • Can you point me at the source of the requirement that built-in MAC must be globally unique? Does the IEEE enforce this legally, for example? I am talking about a built-in MAC for a device that will be installed in homes.
    – Kris Braun
    Jul 19 '16 at 22:09
  • They are supposed to be a universally unique identifier, UUID, AFAIK. Nov 19 '16 at 1:23
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 14 '17 at 21:47
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I think the below link answers your question nicely.

http://www.howtogeek.com/228286/how-is-the-uniqueness-of-mac-addresses-enforced/

The Question SuperUser reader JellicleCat wants to know how the uniqueness of MAC addresses is enforced:

Again and again I have read that devices are uniquely identified by their MAC addresses, but how are MAC addresses determined? And do they tell us anything about the devices they represent?

How is the uniqueness of MAC addresses enforced?

The Answer SuperUser contributor DavidPostill has the answer for us:

How are MAC Addresses Determined?

Vendors are given a range of MAC Addresses that can be assigned to their products by the IEEE (Institute of Electrical and Electronics Engineers). MAC Address are assigned to Vendors in various sized blocks as appropriate.

The IEEE offers Registration Authority programs or registries which maintain lists of unique identifiers under standards and issue unique identifiers to those wishing to register them. The IEEE Registration Authority assigns unambiguous names to objects in a way which makes the assignment available to interested parties. You can go to Vendor/Ethernet/Bluetooth MAC Address Lookup and Search to find the vendor given a particular MAC Address or find a MAC Address Range given to a vendor.

Devices are Uniquely Identified by Their MAC Addresses

This is incorrect. Devices are not uniquely identified by their MAC addresses.

In the past vendors have intentionally or by mistake assigned the same MAC Address to multiple devices. It is possible to change the MAC Address presented by most hardware to the OS, an action often referred to as MAC spoofing: MAC spoofing is a technique for changing a factory-assigned Media Access Control (MAC) Address of a network interface on a networked device. The MAC Address is hard-coded on a network interface controller (NIC) and cannot be changed. However, there are tools which can make an operating system believe that the NIC has the MAC Address of a user’s choosing. See Also: Are MAC Addresses Unique When Coming Out of the Factory? and MAC Address Recycling?

Duplicate MAC Addresses

Manufacturers re-use MAC Addresses and they ship cards with duplicate addresses to different parts of the United States or the world so that there is only a very small chance two computers with network cards with the same MAC Address will end up on the same network. MAC Addresses are ‘burned’ into the Network Interface Card (NIC) and cannot be changed. See ARP and RARP on how IP addresses are translated into MAC Addresses and vice versa. In order for a network device to be able to communicate, the MAC Address it is using must be unique. No other device on that local network subnet can use that MAC Address. If two devices have the same MAC Address (which occurs more often than network administrators would like), neither computer can communicate properly. On an Ethernet LAN, this will cause a high number of collisions. Duplicate MAC Addresses on the same LAN are a problem. Duplicate MAC Addresses separated by one or more routers is not a problem since the two devices won’t see each other and will use the router to communicate. Source: Media Access Control

Further Reading

FAQs: The Registration Authority

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  • That's the page where I read earlier that vendors do reuse MAC addresses. So the practice seems to exist, just not sure if there are consequences.
    – Kris Braun
    Jul 20 '16 at 2:53
  • Not sure it would necessarily create collisions. Level2 broadcasts for sure, collisions, not so much. Anyway, everyone agrees that two interfaces with the same MAC address on the same network segment is a really bad idea. Dec 19 '16 at 14:49
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    Duplicate MACs cause "MAC flapping". The switches keep updating where they think the MAC address is based on the incoming packets and both systems will experience intermittent connectivity. Jan 18 '17 at 1:29

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