How are mac address assigned?

[ISAslot]

Are they assembled via different properties of a hardware device?

Are they created and then assigned to the device?

Do they have to be unique?

What happends if two companies created 2 nic's with the same macs which got installed on the same network?

 

[gunrunnerjohn]

MAC address ranges are assigned to companies by the IEEE. The first six comprise the Organizational Unique Identifier (OUI) which identifies the manufacturer (assigned by the IEEE). The last six represent a unique ID number for the NIC.

 

[ISAslot]

O I C 😀

Thanks John

 

[chsh1ca]

Originally posted by: gunrunnerjohn
The last six represent a unique ID number for the NIC.

Theoretically true, while not factually true.
The IDs are rarely duplicated (it really depends on the raw number of available combinations for each vendor), but it does happen. I've seen firsthand dell workstations that have caused MAC conflicts because of an identical MAC address on the card. This was overcome by using an alternate NIC we had lying around, but don't believe that they are truly unique. Manufacturers build lots of NICs, and there's a limited number of IDs for each manufacturer. 🙂

 

[cmetz]

Some super-cheap manufacturers made entire runs of boards with the same host-part in the MAC, assuming nobody'd notice.

ISAslot, conflicts must be handled gracefully by the network protocols and OSs. IP will do so as a protocol, but some OSs are unlikely to like it. In a switched Ethernet environment you'll have a lot of trouble because the switch will learn that the MAC is down one port... no, it's down another... no, back to the first. And at any given time, you don't know which port or both is going to get traffic destined to that MAC. My guess is that in practice, with an Ethernet switch, it just won't work.

Many OSs (either directly or through third-party programs) allow you to manually configure the MAC address. You could certainly do this to try it and see for yourself. You could just as easily do this to fix a MAC conflict if you knew you had one. 10/100 Ethernet cards come free in specially marked boxes of cereal these days, so if you really have a MAC conflict, another solution would just be to replace and destroy one if not both cards (to make sure they doesn't reappear somewhere else in your network, and where there's one duplication there's a good chance there are more from the same manufacturer).

 

[gunrunnerjohn]

I realize that the MAC addresses are sometimes duplicated, since some devices allow you to specify whatever MAC address you like. 🙂 However, I figured that the "pure" version was best for that kind of question. 😀