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 10

  • Open the Menu
  • Navigate to Settings


  • Open Network & Internet 


  • Open Ethernet


  • Click on the listed ethernet connection. (In this case its


  • Address is listed next to Physical address (MAC)


Windows 7

  • Click on the Windows Logo in the bottom left corner.
  • In the search box, type in cmd and press enter


  • In the command prompt, type in getmac 


  • The MAC address is labeled Physical Address 



  • Go to the Apple Menu
  • Select System Preferences


  • Select Network
  • In the left of the "Network" window that opens, click the name of your connection (e.g., Wi-Fi, AirPort, Ethernet, Built-in Ethernet).


  • Click Advanced, and in the sheet that appears, click the Hardware, Ethernet, or AirPort tab.


  • The MAC address is listed under the tab.



In a terminal window, run:

ifconfig -a

The following is part of the output. Look for em1 in the header and its corresponding  ether
The address next to it is the MAC address.

em1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet  netmask  broadcast
        inet6 fe80::49b6:5ac0:dce8:d1ab  prefixlen 64  scopeid 0x20<link>
        ether 78:2b:cb:48:f7:67  txqueuelen 1000  (Ethernet)
        RX packets 2002985368  bytes 1138703982857 (1.0 TiB)
        RX errors 0  dropped 212148  overruns 0  frame 0
        TX packets 1785113005  bytes 1225863150610 (1.1 TiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0



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.