Every day, adversaries bombard Department of Defense computer networks with scanning traffic in order to gather information about the target network. This reconnaissance is typically a precursor to attacks designed to access data, exfiltrate information, or plant malware in order to gain a military advantage. One specific reconnaissance tool, traceroute, is used to map the network topology of a target network. We implement an active network defense tool, dubbed DeTracer, that seeks to thwart network mapping attacks through the use of deception. We deploy DeTracer in several environments, including the Internet, to demonstrate that an attacker attempting to map a target network using traceroute probes can be presented with a false network topology of the defenders choosing. Our experiments show that a defender can present an adversary with a credible false network topology. We are able to deceive all types of incoming traceroute probes, present a complex false network topology on a per source and destination basis, and deploy our deception scheme without disrupting service to the real production infrastructure on our network.