MAC addresses: the name tag on every network card

The simple idea

Every device that connects to a network, by cable or Wi-Fi, has a network interface, and every interface has a MAC address (Media Access Control address). It is a bit like a name tag sewn into the device at the factory. It looks like this:

3C:22:FB:9A:41:07

That is 48 bits written as six pairs of hexadecimal digits (0 to 9 and A to F). The first half usually identifies the manufacturer; the second half is that maker's number for the individual card.

MAC address or IP address?

A good way to remember it: the IP address is the postal address that gets a parcel to the right town and street; the MAC address is the name on the door used for the final hand-over inside the building.

How the two work together

When your laptop wants to send something to the printer at 192.168.1.30, it first needs the printer's MAC address. It shouts a short question to the whole local network: "who has 192.168.1.30?" The printer answers with its MAC address. This question and answer is called ARP (Address Resolution Protocol). Your laptop remembers the answer for a while in its ARP cache.

For anything outside your network, such as a website, the laptop uses the MAC address of the router (the default gateway) instead, and the router passes the data on.

How a switch uses MAC addresses

A switch learns which MAC address lives on which port by watching the traffic, and stores it in a MAC address table. Then it can send each frame only to the right port instead of to everyone. That is why switches are faster and more private than the old hubs they replaced. See the MAC address table for the details.

Real-world example: the "random" Wi-Fi address

Modern phones and laptops often use a private (randomised) MAC address for each Wi-Fi network, so shops and public hotspots cannot track you from place to place by your device's real name tag. It is a good privacy feature. It also explains a common puzzle at work or school: if a network only lets in "approved" MAC addresses, a phone using a random address will be refused until private addressing is turned off for that one network.

Find your MAC address

Windows:        ipconfig /all       (look for "Physical Address")
macOS / Linux:  ip link   (or ifconfig)
See neighbours: arp -a

Try this: run arp -a on your computer. You will see the IP and MAC address of your router and any other devices your computer has talked to recently.