MAC Addresses: The Talk

Computer systems are full of addresses.  Some of them look like regular numbers to the human eye ( and some of them look a little more complex (2bb1:db8:85a3::8a2e:fa53:7b3c).  There are others less complex ( 7255: F13B) but still just as meaningless to the average eye.

Our computer systems need these addresses for the same reason humans need addresses: to find things.  We use addresses daily to find work, home, friends’ houses and school. Having a street name and a number associated with a city and zip code helps us find our way.

We also use addresses within the places we go, though we might not think about it: Gamestop at the Harrisburg mall is up the stairs and to the left, across from Kay Jewelers; the bathroom at my sister’s house is the first door on the left in the upstairs hall; my car keys are on the third shelf of the bookcase in the dining room next to the graphic novels.

One of the key addresses computers use is a MAC address.  The MAC address is designed to be a unique identifier assigned to each network adapter when it’s made and never changed. Modern networks operate on several layers (beyond the scope of this introduction) and the MAC address can be thought of as the physical address of the hardware itself.

MAC Addresses: The Tech

A standard MAC address of the type used in virtually all home networking is part of the MAC-48 or EUI-48 set of standards. It has six sets of two hexadecimal digits separated by colons. The first three sets of the MAC address refer to the OUI (manufacturer) and the second three refer to the NIC (device).  Since the numbers are hexadecimal, this allows for 16,777,216 manufacturers to each create 16,777,216 network cards with unique addresses: a grand total of 281,474,976,710,656 possible unique MAC addresses.

In English:

  1. The MAC address is of the format ##:##:##:##:##:##.
    Those are colons (:) up there separating the number signs.
  2. Each # represents a hexadecimal number.
    I used number signs because a recent survey says letters that stand for numbers is the number N reason that Y out of X people hate math.
  3. Hexadecimal numbers include the digits 0 through 9 and the letters A through F.
    There’s a very technical reason for this.  Trust me.  The important part is that it gives sixteen (0 through 9 is ten, A through F is six more) values to a single hex digit.
  4. Some systems will list the MAC address as ##-##-##-##-##-##.  It’s the same thing.
    Those are dashes (-) up there separating the number signs this time. Don’t ask.

Some example MAC addresses:

  1. 00:02:B9:AA:11:19
    I made this up, but the first three sets of two numbers (00:02:B9) tell us this is some network device by Cisco.
  2. 60-45-BD-63-03-49
    I grabbed this one from the home network.  The first six numbers (60-45-BD) identify it as a Microsoft device.
    This isn’t made up, but is legitimate.  It’s the MAC address for a broadcast. The average home user doesn’t need – and probably doesn’t want – to know about this.

MAC Addresses: The Takeaway

Why do you care?

You can use MAC addresses as one way to limit the people that connect to your home network.

It’s not the most foolproof form of security ever, but if your son is playing Black Ops 2 instead of mowing the lawn perhaps a mysterious network error (in the form of you adding the MAC of his 360 or Playstation to your wireless router’s MAC exclusion list) would be motivational. Maybe the next time the cousin that killed your network downloading pirated movies visits, her laptop mysteriously can’t connect (because you added a list of allowed devices and she isn’t on it). You might want to share your Internet connection with one of your neighbors via wireless but not all of your neighbors (in this case you would use a list of allowed devices, add your neighbor’s device’s MAC address and give her your security pass phrase – then even if she tries to share it with a close, close friend it won’t work).

Now you know what a MAC address is and why you might want that knowledge. The next post will focus on more hands-on material: how to find a device’s MAC address.  After that we’ll get into the real juicy stuff: using those MAC addresses to lock down your network a little bit more.