As of April 2013 almost 95% of the IPv4 address space has been allocated. Yet, the transition to IPv6 is still relatively slow. One reason could be existing “IPv4 reserves” – allocated but unused IPv4 addresses. Knowing how many addresses are actively used is important to predict a potential IPv4 address market, predict the IPv6 deployment time frame, and measure progressive exhaustion after the IPv4 space is fully allocated. Unfortunately, only a fraction of hosts respond to active probes, such as “ping”. We propose a capture-recapture method to estimate the actively used IPv4 addresses from multiple incomplete data sources, including “ping” censuses, network traces and server logs. We estimate that at least 950–1090 million IPv4 addresses are used, which is 36–41% of the publicly routed space. We analyse how the utilisation depends on various factors, such as region, country and allocation prefix length
