Network anomaly detection is an important and dynamic research area. Many Network Intrusion Detection methods and Systems (NIDS) have been proposed in the literature. In this paper, the authors provide a structured and comprehensive overview of various facets of network anomaly detection so that a researcher can become quickly familiar with every aspect of network anomaly detection. They present attacks normally encountered by network intrusion detection systems. They categorize existing network anomaly detection methods and systems based on the underlying computational techniques used. Within this framework, they briefly describe and compare a large number of network anomaly detection methods and systems.