When you power-on your computer it will broadcast to the world a list the list of WiFi access-points you've got cached on your computer, the previous IP address you used (requested by DHCP), your NetBIOS name, your login ID, and a list of servers (via NetBIOS request) you want connections to.
Even if you then establish a VPN connection to hide everything else, you've already broadcasted this information to everyone on the local network.
The FERRET tool gathers this broadcasted information and correlates it. It demonstrates how much you expose to hackers.
Source: erratasec.com
