In this talk, we explore the degree to which commercial strategies, peering disputes, network failures, misconfiguration, and occasionally, malicious intent, lead to a partitioning of Internet topology. Specifically, we present a three-year study of the differences in Internet provider reachability. We focus on "dark address space," or the range of topology accessible from one provider, but unreachable via one or more competitor networks. We present active and passive measurements of these differences on time scales ranging from several seconds to multiple months.