MAC Addresses

MAC is short for Media Access Control. The MAC address is the hardware address that uniquely identifies each network device on the Caltech network. The MAC address is also sometimes called the ethernet address.

More details are available below about what a MAC Address really is. Please note that a MAC Address is not the same thing as an IP Address!

Windows XP, 2000, and NT

  1. Click on the Start Menu then select Run.
  2. Type cmd then click OK.
  3. In the Command Prompt box, type ipconfig /all then hit Enter.
  4. Note the Physical Address, this is also the MAC address.

Windows ME, 98, and 95

  1. Click on the Start Menu then select Run.
  2. Type winipcfg then click OK.
  3. In the drop down menu, select the item corresponding to your network card (ignore the PPP Adapter).
  4. Note the Adapter address, this is the MAC address.


In MacOS 10.2 and newer, there are two methods of finding your computer's MAC address. In versions earlier than 10.2, only the first method is available.

Method 1:

  • Go to the Apple Menu
  • Select System Preferences
  • Select Network
  • Be sure that the pulldown list is at Show: Built-In Ethernet
  • Select the TCP/IP tab
  • The MAC address will be shown as Ethernet Address

Method 2 (works with 10.2 and newer):

  • Go to the Apple Menu
  • Select About This Mac
  • Select More Info
  • Select the System Profile tab
  • Under Network Overview: Built-In, the MAC address will be shown as Ethernet Address

MacOS 9.2 running TCP/IP (Open Transport)

  1. Select the Open Transport Control Panel from the Control Panels folder in the Apple or open the file directly with this path:
  2. [Local Macintosh Hard Disk::System Folder::Control Panels::TCP/IP]
  3. The control panel will look like this:
  4. Get information on the TCP/IP Control Panel
  5. From the Filemenu heading, select Get InformationOR do the following shortcut:
  6. The string of characters which are highlighed in yellow (following the heading Hardware Addressin the picture above) is the MAC address of your computer. Your computer's number should be different than the number listed here.


ifconfig -a

The following is the resulting output. The MAC address appears on the same line as the name of each device (labeled HWaddr). In this example, eth0 is the name of the device that has a MAC address. lo will not have a MAC address as it is a loop back address only. If you have more than one device connected to the network, it will be listed as well, with a different MAC address.

				eth0 Link encap:Ethernet HWaddr 00:90:27:2A:C4:65
   inet addr: Bcast: Mask:
   RX packets:96182694 errors:0 dropped:0 overruns:0 frame:2
   TX packets:137175658 errors:0 dropped:0 overruns:0 carrier:0
   collisions:0 txqueuelen:100
   Interrupt:14 Base address:0x6000
lo Link encap:Local Loopback
    inet addr: Mask:
    RX packets:5738011 errors:0 dropped:0 overruns:0 frame:0
    TX packets:5738011 errors:0 dropped:0 overruns:0 carrier:0
    collisions:0 txqueuelen:0


At the prompt, typearpand the machine name as shown:

					  {shada:2194} arp shada
	  shada ( at 8:0:20:b9:59:e6 permanent published

You can generalize this to

					  arp `uname -n`

Note that backticks surround theuname -ncommand.


At the prompt, typeifconfig -aas shown. Note that the MAC address is also called the ethernet address abbrievated toetherby this command.

				accord# ifconfig -a
le0: flags=63 

More Details about What A MAC Address Really Is

In order for a device to connect to a network, the device needs to have a unique ID. This ID is similar to a street address for a house; there should be no two devices with the same address. The address is used to send data from one device to another device.

In the case of Ethernet packets, the following information is stored in each data packet on a local network:

  • Mac Control -- This contains the protocol control information
  • Destination MAC Address -- Where the packet is to be delivered to
  • Source MAC Address -- Where the packet is being sent from
  • LLC PDU -- The actual data in the ethernet packet
  • CRC -- Error detecting code

At Caltech, we use the MAC address to keep track of what device is using what IP address and who/what device is using the network. This is important information to prevent and/or resolve network problems caused by two devices using the same address. When two devices use the same address, network performance is poor for those devices, or one of the devices' network connectivity no longer works at all.

References (these are sites outside of Caltech and IMSS):