Randy's Work Blog

As ISPs face IPv4 address scarcity they increasingly turn to network address translation (NAT) to accommodate the ad- dress needs of their customers. Recently, ISPs have moved beyond employing NATs only directly at individual cus- tomers and instead begun deploying Carrier-Grade NATs (CGNs) to apply address translation to many independent and disparate endpoints spanning physical locations, a phe- nomenon that so far has received little in the way of empiri- cal assessment. In this work we present a broad and system- atic study of the deployment and behavior of these middle- boxes. We develop a methodology to detect the existence of hosts behind CGNs by extracting non-routable IP addresses from peer lists we obtain by crawling the BitTorrent DHT. We complement this approach with improvements to our Netalyzr troubleshooting service, enabling us to determine a range of indicators of CGN presence as well as detailed insights into key properties of CGNs. Combining the two data sources we illustrate the scope of CGN deployment on today’s Internet, and report on characteristics of commonly deployed CGNs and their effect on end users.