How can a MAC address be unique?

[J_T]

The first six digits of a MAC address is an OUI. If one manufacturer makes more than 16,777,216 devices using the same OUI, some devices will have a duplicate MAC address.

How do manufacturers avoid this problem? Do they buy a new OUI from IEEE?

[Leo Laohoo · Accepted solution]

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@J_T wrote:

Do they buy a new OUI from IEEE?

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Yes. 

If a company acquires another company, they also get that OUI.