Network based intrusion detection are the most deployed IDS. They frequently rely on signature matching detection method and focus on the security of low level network protocols. Because of the large number of false positives from one side, and the incapacity to detect some attack types from another side, IDS must allow more interest to the monitoring of application level protocols.We propose in this paper a combination of pattern matching and protocol analysis approaches. While the first method of detection relies on a multipattern matching strategy, the second one benefits from an efficient decision tree adaptative to the network traffic characteristics.