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.
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**.
1. Unless you choose to [[hide that you are connecting to the Tor network|anonymous_internet/tor]], someone who monitors your Internet connection can know that. In this case, your MAC address can **reveal that you are a Tor user**.
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.
On some network interfaces, **MAC address anonymization is impossible** due to limitations in the hardware or in Linux. Tails temporarily disables such network interfaces. You might disable MAC address anonymization to be able to use them.
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.