Physical communication links are determined between devices in a network of devices without human input. A network topology engine determines the physical communication links between devices in the network and constructs a network topology based on the determined physical communication links. The network topology engine infers the presence of each physical communication link from network traffic that is currently flowing in the network. The network topology engine collects reachability information from the network devices included in the network that are already stored by the network devices, such as device identification entries included in address resolution protocol tables and media access control address tables. The network topology engine populates a flow graph of potential physical links between network interfaces that is weighted based on the collected reachability. The network topology engine then selects the highest weighted potential physical links between network interfaces to be the actual physical links of the network.