Few studies so far have examined the nature of reachability policies in enterprise networks. A better understanding of reachability policies could both inform future approaches to network design as well as current network configuration mechanisms. In this paper, we introduce the notion of a policy unit, which is an abstract representation of how the policies implemented in a network apply to different network hosts. We develop an approach for reverse-engineering a network’s policy units from its router configuration. We apply this approach to the configurations of five productions networks, including three university and two private enterprises. Through our empirical study, we validate that policy units capture useful characteristics of a network’s policy. We also obtain insights into the nature of the policies implemented in modern enterprises. For example, we find most hosts in  these networks are subject to nearly identical reachability policies at Layer 3.