Since its inception, Software Defined Network (SDN) has made itself a very appealing architecture for both Data Center and Wide Area networks by offering more automated control through programmability and simplified network operations and management with its centralized control plane. However, extensive rollout of SDN in the production environment requires thorough validation. Thus, there is a compelling need for SDN emulators that facilitate experimenting with new SDN-based technologies (e.g., SDN-based routing and traffic engineering schemes). Valuable insights on these technologies can be gained from real trace-driven experiments on an emulator platform. Accordingly, network operators can gain confidence in these technologies without jeopardizing their infrastructures and businesses. Mininet, the de facto standard SDN emulator, allows users to emulate an OpenFlow-based SDN on a single server. Due to the physical resource limitations of a single machine, Mininet fails to scale with large network size and high traffic volume. To address these limitations, we developed Distributed OpenFlow Testbed (DOT), a highly scalable emulator for SDN. DOT distributes the emulated network across multiple physical machines to scale with large network sizes and high traffic volumes. It also provides guaranteed compute and network resources for the emulated components (i.e., switches, hosts and links). Moreover, DOT can emulate a wider range of network services compared to other publicly available SDN emulators and simulators.