Capturing Ghosts: Predicting The Used Ipv4 Space By Inferring Unobserved Addresses

The pool of unused routable IPv4 prefixes is dwindling, with less than 4% remaining for allocation at the end of June 2014. Yet the adoption of IPv6 remains slow. We demonstrate a new capture-recapture technique for improved estimation of the size of "IPv4 reserves" (allocated yet unused IPv4 addresses or routable prefixes) from multiple incomplete data sources. A key contribution of our approach is the plausible estimation of both observed and unobserved-yet-active (ghost) IPv4 address space. This significantly improves our community's understanding of IPv4 address space exhaustion and likely pressure for IPv6 adoption. Using "ping scans", network traces and server logs we estimate that 6.3 million / 24 subnets and 1.2 billion IPv4 addresses are currently in use (roughly 60% and 45% of the publicly routed space respectively). We also show how utilisation has changed over the last 2-3 years and provide an up-to-date estimate of potentially-usable remaining IPv4 space.