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MAC Addresses

March 29, 2008

Every network card has a built-in, unique ID, known as its Media Access Control (Mac) address. This address is fundamental to the operation of all mainstream networking technologies, as it’s the one address that can uniquely identify a specific card – and thus a specific computer or other device – on the network. There are other addressing schemes that are put in place when you’re setting up a practical network, but these addressing schemes are not the ones used to pump data around a network; that’s the job of the Mac address. In general, it’s not necessary to worry about the Mac address of your NIC during installation and setup, because the software parts of your setup that do need to know can easily find it out by asking the NIC. It’s worth knowing, however, that the Mac address exists because some configuration and diagnostic tools will display it, and that it can also be used for network security. Network operating systems such as Microsoft Windows NT and Novell NetWare allow you to restrict the locations from which a user can log in by specifying the Mac addresses of the relevant workstations.

In their raw form, Mac addresses are 48-bit binary numbers that look like this: 000000001110000010011000000000010000100100001110

To make these numbers easier to read and document, they are usually written in hexadecimal (base 16) format, which looks like this:

00 E0 98 01 09 0E

Being 48 bits long allows for a possible 281,474,976,710,656 (or 2 to the 48th power) Mac addresses.

Because the pool of possible Mac addresses is so large, the IEEE has been tasked with ensuring that no two NIC cards ever share the same Mac address. To achieve this, NIC manufacturers are assigned one or more start addresses – the top 24 bits of the Mac address, which IEEE calls the Organizationally Unique Identifier (OUI) and the manufacturer then uses the remaining 24 bits to give each card it produces a truly unique address.

How do you find out the Mac address of your network card? Open a Command Prompt (“cmd” under the Run command – Windows Key+R), and type ipconfig/all. The Mac address will be listed as the Physical Address, in hexadecimal format.

Important Note: If you change the NIC card in your computer, you effectively change your computer’s Mac address.

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