  One challenge in understanding the evolution of Internet infrastructure is
the lack of systematic mechanisms for monitoring the extent to which allocated
IP addresses are actually used. In this paper we try to advance the science of
inferring IPv4 address space utilization by analyzing and correlating results
obtained through different types of measurements. We have previously studied an
approach based on passive measurements that can reveal used portions of the
address space unseen by active approaches. In this paper, we study such passive
approaches in detail, extending our methodology to four different types of
vantage points, identifying traffic components that most significantly
contribute to discovering used IPv4 network blocks. We then combine the results
we obtained through passive measurements together with data from active
measurement studies, as well as measurements from BGP and additional datasets
available to researchers. Through the analysis of this large collection of
heterogeneous datasets, we substantially improve the state of the art in terms
of: (i) understanding the challenges and opportunities in using passive and
active techniques to study address utilization; and (ii) knowledge of the
utilization of the IPv4 space.
