What is a MAC address?

Every network interface — wired or Wi-Fi — has a [[!wikipedia MAC address]], which is a serial number assigned to each interface at the factory by the vendor. MAC addresses are used on the local network to identify the communications of each network interface.

While your IP address identifies where you are on the Internet, your MAC address identifies which device you are using on the local network. MAC addresses are only useful on the local network and are not sent over the Internet.

Having such a unique identifier used on the local network can harm your privacy. Here are two examples:

1. If you use your laptop to connect to several Wi-Fi networks, the
same MAC address of your Wi-Fi interface is used on all those local networks. Someone
observing those networks can recognize your MAC address and **track your
geographical location**.

<p>To learn how Tails implements MAC address anonymization, see our
[[design documentation about MAC address anonymization|contribute/design/MAC_address]].</p>

Here are a few examples:

An adversary on the local network can only see encrypted connections to the Tor network.

Your MAC address is not sent over the Internet to the websites that you are visiting.

However, as [[discussed above|doc/first_steps/welcome_screen/mac_spoofing#what-is-mac-address]], disabling MAC address anonymization makes it possible for someone to track your geographical location. If this is problematic, consider using a different network interface, like a [[USB Wi-Fi adapter|doc/anonymous_internet/no-wifi#wifi-adapters]], or moving to another network.

**Using a public computer**, for example in an Internet café or a library. This computer is regularly used on this local network, and its MAC address is not associated with your identity. In this case, MAC address anonymization can make it impossible to connect. It can even **look suspicious** to the network administrators to see an unknown MAC address being used on that network.

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Select the **Don't anonymize MAC addresses** option.

Other considerations

Other means of surveillance can reveal your geographical location: video surveillance, mobile phone activity, credit card transactions, social interactions, etc.

While using Wi-Fi, anybody within range of your Wi-Fi interface can see your MAC address, even without being connected to the same Wi-Fi access point.

When using mobile phone connectivity, such as 3G or GSM, the identifier of your SIM card (IMSI) and the serial number of your phone (IMEI) are always revealed to the mobile phone operator.