We consider a problem faced by an intelligent roadside unit (RSU) monitoring a roadway by a video camera. Suppose the RSU notices that a particular car in its visual field needs to execute a specific evasive maneuver to avoid danger. It would like to send a packet addressed to that particular car with this suggestion. The problem is that while all the cars are communicating with the RSU, the RSU does not know which car in the video is associated with what IP address. So, it does not know which IP address to send the packet to. Indeed, the problem of matching addresses with cars in the visual field is a fundamental open problem. We provide an active solution employing dynamic watermarking that was originally developed for the security of cyber-physical systems. This technique calls for a car to superpose a small random excitation onto its actuation commands for steering angle or throttle/brake positions. The car sends this random waveform to the RSU in a packet containing its IP address. By signal processing of the video stream of a car at the RSU it can verify whether it matches with the waveform in the packet and thereby associates that the IP address of the packet with that car in the visual field. The RSU thereby determines which IP address is associated with which car in its visual field. We present two demonstrations of performance. We demonstrate experimental results on a laboratory transportation automated vehicles, a vision system, and a network, as well as on the field with two passenger sedans in practice. The results demonstrate that employing the dynamic watermarking method enables an RSU to distinguish the communication of a target vehicle from that of other IP addresses of nearby vehicles.