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?"