To track individual computers online, just identify how their internal clocks keep time.
Subtle differences in hardware make every computer clock slightly skew from true time, despite use of standard protocols like NTP to synchronize with standard time. Researchers can determine the clock skew of a particular computer by analyzing the TCP/IP packets it sends. They found they could identify an individual machine with this fingerprint, regardless of where on the Net the machine is connected or which operating system it runs. But they say the technology could eventually be defeated by a slight redesign in the core TCP/IP stacks. caida.org/outreach/papers/2005/fingerprinting/