Sunday, May 23, 2010

Basics of MAC address ( Media Access Control )

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Ethernet / NIC / Physical / LAN / BIA / MAC Addressing (alieases of MAC address)

MAC address :- short for Media Access Control

The MAC address is used by switches to send frames to the proper destination, as you'll see in the LAN Switching section. The entire MAC address is a 48-bit address that looks  a little something like :
aa-bb-cc-11-22-33

That MAC address actually has two parts, the first being the Organizationally Unique Identifier (OUI). in the ex "aa-bb-cc" is the OUI. The OUI is assigned to hardware vendors by the Institure of Electrical and Electronics Engineers (IEEE). A given OUI is assigned to one and only one vendor.

The second half of the MAC address is a value not yet used by that particular vendor. Looking at the MAC address example given earlier, we now know that:

 -  the OUI is aa-bb-cc
 -  the vendor has not yet used 11-22-33 with that particular OUI, so the vendor is doing so now. If a single vendore like CISCO has two different OUIs assigned, same second half of MAC address can be used for two different OUIs.

The MAC address is sometimes called the physical address because it physically esists on the network card. The address is burned into the card, giving it yet another name - the Burned-In Address (BIA).

As with IP addresses, we have broadcast and multicast MAC addresses. It's a good idea to be able to identify these addresses, and here's how to do it!

The broadcast MAC address is the "all-Fs" address : ff-ff-ff-ff-ff-ff (or FF-FF-FF-FF-FF-FF, as case does not matter in hexadecimal)

The is a range of multicast MAC addresses, and the first half of a multicast MAC address is always "01-00-5e". The seond half of a multicast MAC address will fall in the range 00-00-00 through 7F-FF-FF. watch out that 1!!!
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