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While imaging some computers today, I started to wonder... what if two LAN MAC addresses on two different computers matched?... That would cause some problems. I later came to understand that the MAC address' 48-bit address space contains potentially 248 or 281,474,976,710,656 possible MAC addresses. [ in other-words, a lot of networking devices ]

How are these MAC addresses determined?

Will we ever run out of them? ( I know the second question is speculation, but there are a lot of devices that require a mac addresses...)

Do MAC addresses get recycled?

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I've read stories about how the manufacturing hardware would accidentally stop incrementing the MAC addresses, and a batch of NICs would have identical addresses; then someone building a large network would purchase the entire batch and find a nasty surprise... –  grawity Nov 12 '12 at 23:36

3 Answers 3

up vote 12 down vote accepted
  1. The MAC is broke into two parts; the OUI (1st 24 bits) and the device ID (last 24 bits). The IEEE controls and allots the OUI portion; so if you got into the business of making your own NICs you would have to register with the IEEE and get your own unique OUI. The device ID portion is administered freely by the company.

  2. We could very well run out, but a MAC address only needs to be unique across an individual layer 2 network.

  3. The OUI portion is not something that is recycled, if you are poking around in your switches MAC address table you can tell by the first 24 bits who made the device, such as Cisco, Google "OUI lookup".

I am sure that individual manufacturers probably recycle their MACs, just so as long as those two NICs never end up on the same layer 2 network then it would be fine.

Network administrators can even choose to use locally administered MAC addresses.

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What sort of recycling policy will they take on? 2^16 is ~16 million, do they really need to recycle MACs? –  Hengjie Nov 13 '12 at 5:58
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@Hengjie: 16 million is not that much. For example, Cisco shipped over 2 million servers in just the first quarter of 2012 ( news.techworld.com/storage/3361179/… ). So a big manufacturer will need to recycle device IDs (or get more OUIs). –  sleske Nov 13 '12 at 9:47

A mac address is generated by a group called INTERNic and sold to NIC vendors in blocks. the first 24B are a vendor ID, and the last 24B are the individual address.

duplicate MACS happen since vendors only want to buy a minimal number of addresses, but duplication is only a problem if the two nics are on the same LAN. if the two MACs are on differant LANs then they will never see each other and never collide.

MACs are Layer 2 addresses and help a data frame move about in a LAN. when that data frame is destined for another network, it addresses its frame to the LAN local router port, and when moved from one LAN to another, the packet is put in a new frame for that LAN using the routers LAN local port as the sender, and the exit routers local port for the destination.

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InterNIC handles DNS, not MAC addresses. The two uses of the abbreviation "NIC" are different and unrelated: Network Interface Card vs. Network Information Center. –  Wyzard Nov 13 '12 at 1:55

In answer to your questions:

  • True - MAC addresses are NOT unique.
  • however it is improbable that the same MAC addresses will exist on the same LAN
  • They are set up on the network interface card when built

Here is the Wikipedia link: http://en.wikipedia.org/wiki/MAC_address

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