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A critical problem faced by a network intrusion detection system (NIDS) is that of ambiguity. The NIDS cannot always determine what traffic reaches a given host nor how that host will interpret the traffic, and attackers may exploit this ambiguity to avoid detection or cause misleading alarms. We present a lightweight solution, active mapping, which eliminates TCP/IP-based ambiguity in a NIDS analysis with minimal runtime cost. Active mapping efficiently builds profiles of the network topology and the TCP/IP policies of hosts on the network; a NIDS may then use the host profiles to disambiguate the interpretation of the network traffic on a per-host basis. Active mapping avoids the semantic and performance problems of traffic normalization, in which traffic streams are modified to remove ambiguities. We have developed a prototype implementation of active mapping and modified a NIDS to use the active mapping-generated profile database in our tests. We found wide variation across operating systems' TCP/IP stack policies in real-world tests (about 6700 hosts), underscoring the need for this sort of disambiguation.