Intrusion Prevention Systems (IPS), also known as Intrusion Detection and Prevention Systems (IDPS), are network security appliances that monitor network and/or system activities for malicious activity. The main functions of intrusion prevention systems are to identify malicious activity, log information about said activity, attempt to block/stop activity, and report activity.  Intrusion prevention systems are considered extensions of intrusion detection systems because they both monitor network traffic and/or system activities for malicious activity.
The main differences are, unlike intrusion detection systems, intrusion prevention systems are placed in-line and are able to actively prevent/block intrusions that are detected.  More specifically, IPS can take such actions as sending an alarm, dropping the malicious packets, resetting the connection and/or blocking the traffic from the offending IP address.  An IPS can also correct Cyclic Redundancy Check (CRC) errors, unfragment packet streams, prevent TCP sequencing issues, and clean up unwanted transport and network layer options. 2]  Contents [hide] 1 Classifications 2 Detection methods 3 See also 4 References 5 External Links Classifications Intrusion prevention systems can be classified into four different types: Network-based Intrusion Prevention (NIPS): monitors the entire network for suspicious traffic by analyzing protocol activity. Wireless Intrusion Prevention Systems (WIPS): monitors a wireless network for suspicious traffic by analyzing wireless networking protocols.
Network Behavior Analysis (NBA): examines network traffic to identify threats that generate unusual traffic flows, such as distributed denial of service (DDoS) attacks, certain forms of malware, and policy violations. Host-based Intrusion Prevention (HIPS): an installed software package which monitors a single host for suspicious activity by analyzing events occurring within that host. Detection methods The majority of intrusion prevention systems utilize one of three detection methods: signature-based, statistical anomaly-based, and stateful protocol analysis. 3]  Signature-based Detection: This method of detection utilizes signatures, which are attack patterns that are preconfigured and predetermined. A signature-based intrusion prevention system monitors the network traffic for matches to these signatures. Once a match is found the intrusion prevention system takes the appropriate action. Signatures can be exploit-based or vulnerability-based. Exploit-based signatures analyze patterns appearing in exploits being protected against, while vulnerability-based signatures analyze vulnerabilities in a program, its execution, and conditions needed to exploit said vulnerability.
Statistical Anomaly-based Detection: This method of detection baselines performance of average network traffic conditions. After a baseline is created, the system intermittently samples network traffic, using statistical analysis to compare the sample to the set baseline. If the activity is outside the baseline parameters, the intrusion prevention system takes the appropriate action. Stateful Protocol Analysis Detection: This method identifies deviations of protocol states by comparing observed events with “predetermined profiles of generally accepted definitions of benign activity. ”