An anonymous reader writes: I've recently noticed that the reverse DNS name given to my IP from my ISP contains my mac address. It seems to me that regardless of IP address/dhcp logs that this could serve as a permanent unique identifier for a person. How many other ISPs do this? Are we clearing our google cookies periodically for nothing? Is this a privacy hole that should be closed up? I can see the ISPs internally being able to recognize their clients uniquely, but to the rest of the Internet is it a security violation for people to be tracked by an unchanging hostname?