A Networking Primer: What's a MAC Address?

By Joseph Moran | Posted September 30, 2009

When you think about networking, IP addresses are probably the first things that come to mind. But there’s another type of network address called a MAC address that actually forms the foundation upon which IP address communication is built, at least where local area networks are concerned.

What Is a MAC Address?

A MAC address, sometimes referred to as a hardware address or physical address, is an ID code that’s assigned to a network adapter or any device with built-in networking capability (say, a printer). While an IP address can potentially be assigned to any device, a MAC address is “burned in” to a given device from the factory. A MAC address takes the form of six pairs of hexadecimal digits, usually separated by colons or dashes, and will look something like this—01:1F:33:69:BC:14. (Hexadecimal digits can only include the numbers 0-9 and letters A-F.)

The first three pairs of digits in the MAC address are called the OUI (Organizational Unique Identifier), which identifies the company that manufactured or sold the device. For example, a MAC address that begins with 00:1F:33 denotes a Netgear product. The last three pairs of digits are specific to the device and can be more or less considered a serial number of sorts. Together, the two parts of the MAC address form an ID that’s unique to a particular device. 

To understand how MAC addresses are used, first consider that when you type www.smallbusinesscomputing.com into your Web browser, it can’t get there until a DNS (Domain Name Service) server looks up the corresponding IP address for the Web site allowing a connection to take place. While MAC addresses don’t have any real significance on the Internet, they’re used in a similar way on a local area network.

Given that IP addresses can’t be permanently assigned to a device—after all, a particular address can belong to one computer today and another one tomorrow— MAC addresses allow communication between devices on a local network by making it possible to reliably distinguish one computer from another. Just as DNS matches a Web site name to an IP address on the Internet, a technology called ARP (Address Resolution Protocol) matches an IP address to the corresponding MAC address of a specific device to which that IP address is currently assigned.

What’s My MAC Address?

Finding the MAC address on a Windows PC is easy via the command-prompt utility called IPCONFIG. To run it in Windows Vista or Windows 7, click the Start button then type “cmd” (no quotes) into the search box and hit Enter. (In XP, click Start, then Run, then type “cmd” and hit Enter.) Then from the command prompt, type ipconfig /all and you’ll see the system’s MAC address listed next to “Physical Address.”  

Finding the MAC address on a Mac (no relation) is trickier and varies slightly by operating system version. Select System Preferences from the dock, then select Network, and depending on whether you’re using a wired or wireless connection, click on either Ethernet or AirPort. Finally look for either the “Ethernet ID” or “AirPort ID” (you may need to click an Advanced button to find them).  

For other network devices like external adapters and other non-PC devices, the MAC address is generally displayed on a sticker at the back or bottom of the unit. In the case of network-enabled printers with built-in control panels (as most have these days), you can find the MAC address by visiting the unit’s Network menu or printing out a configuration page (check documentation re: how to do this).

MAC Address Tips

Since MAC addresses are used behind the scenes, you generally don’t need to pay a lot of attention to them, but here are three instances when being familiar with a device’s MAC address can be worthwhile.

Identifying Unknown Devices

You may recall from earlier that the first part of a MAC address -- the OUI -- indicates a device’s manufacturer or vendor. When you encounter a MAC address but don’t know the device it belongs to (for example, routers sometimes list the IP and MAC addresses for attached devices without providing any other information about them) identifying the OUI can help you identify the device (or at least narrow it down). Two good sites to look up a MAC address OUI are here and here. Remember, you only need to enter the first six digits of the MAC address to determine the OUI.

DHCP Reservations

DHCP reservations are handy when you want to automatically assign a consistent IP address to a device like a printer or NAS unit without having to visit the device to configure an address manually. Whether your DHCP server resides in a router, or a Windows 200x or other type of server, you’ll need to know a device’s MAC address to create a DHCP reservation for it.

Wake on LAN

Wake-on-LAN offers a way to send a remote signal to revive a computer that’s in a power-saving mode (e.g. sleep/standby or hibernation) or even turned off. (Almost all systems built within the last several years support Wake-On-LAN, though you may need to turn the feature on in the system BIOS.) While a dormant computer can’t be contacted via IP address--technically, it doesn’t have one—you can reach it by MAC address. You can download a free Wake-on-LAN utility (for Windows) here.

Joe Moran spent six years as an editor and analyst with Ziff-Davis Publishing and several more as a freelance product reviewer. He's also worked in technology public relations and as a corporate IT manager, and he's currently principal of Neighborhood Techs, a technology service firm in Naples, Fla. He holds several industry certifications, including Microsoft Certified Systems Engineer (MCSE) and Cisco Certified Network Associate (CCNA).

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