Decius wrote: "Steven Bellovin is at it again." Abstract: There have been many attempts to measure how many hosts are on the Internet. Many of those endpoints, however, are NAT boxes (Network Address Translators), and actually represent several different computers. We describe a technique for detecting NATs and counting the number of active hosts behind them. The technique is based on the observation that on many operating systems, the IP headers ID field is a simple counter. By suitable processing of trace data, packets emanating from individual machines can be isolated, and the number of machines determined. Our implementation, tested on aggregated local trace data, demonstrates the feasibility (and limitations) of the scheme. A Technique for Counting NATted Hosts [PDF] |
