NAT usage in residential broadband networks

Many Internet customers use network address translation (NAT) when connecting to the Internet. To understand the extend of NAT usage and its implications, we explore NAT usage in residential broadband networks based on observations from more than 20,000 DSL lines. We present a unique approach for detecting the presence of NAT and for estimating the number of hosts connected behind a NAT gateway using IP TTLs and HTTP user-agent strings. Furthermore, we study when each of the multiple hosts behind a single NAT gateway is active. This enables us to detect simultaneous use. In addition, we evaluate the accuracy of NAT analysis techniques when fewer information is available. We find that more than 90% of DSL lines use NAT gateways to connect to the Internet and that 10% of DSL lines have multiple hosts that are active at the same time. Overall, up to 52% of lines have multiple hosts. Our findings point out that using IPs as host identifiers may introduce substantial errors and therefore should be used with caution.