A detection and response system including a set of algorithms for detection within a stream of normal computer traffic a subset of TCP packets with one IP Source Address (SA), one Destination Port (DP), and a number exceeding a threshold of distinct Destination Addresses (DA). There is efficient use of a lookup mechanism such as a Direct Table and Patricia search tree to record sets of packets with one SA and one DP as well as the set of DA values observed for the given SA, DP combination. The existence of such a subset and the header values including SA, DP, and multiple DAs of the subset are reported to a network administrator. In addition, various administrative responses to reports are provided.