An ideal Intrusion Detection System(IDS)should implement unsupervised learning and online learning. Existing methods suffice these two criterions requires too much training time, which would cause a high packet loss rate and is unacceptable. To overcome the difficulty, an intrusion detection method based on improved Self-Organizing Incremental Neural Network(SOINN)and data reduction is presented, which allows online training of network classifiers in an unsupervised fashion. Also, data reduction methods, in...