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Is the MAC address used on the DLL layer unique throughout the world for a device, lets say this computer?

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The MAC address burned into the hardware chip on the network card is supposed to be unique. Here is a database that will allow you to look up which vendors are allocated which mac addresses. On the software level, most OSs (including Windows, Linux, Mac OSX, and BSD to name a few) allow you to change the MAC address that the card uses, which is known as "MAC Spoofing". This does not change the address that's burned into the hardware, it simply lets the OS write a custom MAC address into the packets. You can disable the spoofing at any time to get the original MAC address back.

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Sometimes, not always. Some NIC manufactures reuse MAC addresses on many NICs because its unlikely that two with the same MAC will end-up on the same LAN. That's why they let you change the MAC address; in-case you get two with the same MAC, you can change one. I know people this has happened to.

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what NIC meaning here? –  user39966 Oct 29 '10 at 6:34
    
NIC = Network Interface Card. I guess its lingo that is a little out of date since network interfaces are usually built right on the mother board these days. –  Jay Oct 29 '10 at 12:59
    
Could also mean "Network Interface Controller" which would apply to an onboard adapter. –  ultrasawblade Apr 21 '12 at 20:08

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