Hybrid switching combines the strengths of electronic packet switching and high-speed optical circuit switching and is thus widely believed to be a cost-effective solution for current and future data center networks (DCNs). In designing a hybrid switching DCN, it is essential to know how many resources should be devoted to optical circuit switching and electronic packet switching so the performance requirements in both planes can be satisfied, and, at the same time, objectives such as minimizing the overall system building cost or the power consumption can be realized. In this paper, we introduce BLOC, the Blocking LOss Curve, as a tool to characterize hybrid switching DCNs. With BLOC, the three interacting components in hybrid switching systems-resource allocation, traffic partitioning, and system performance-can be naturally integrated and studied together. We show how BLOC can be used to obtain feasible resource allocation/traffic partition combinations. We also show how system parameters, such as flow arrival characteristics, may affect resource allocation and system costs. Our study reveals that the total amount of resource required to satisfy 95th percentile traffic can be as low as 60% of that required for 99.9th percentile traffic. [ABSTRACT FROM PUBLISHER]