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