An intrusion prevention system (IPS) can inspect traffic and detect any suspicious traffic based on a variety of factors, but it can also actively block such traffic. Although an IDS can detect the same types of suspicious traffic as an IPS, it is only design to alert, not to block. A firewall is only concerned with IP addresses, ports, and protocols; it cannot be used for the signature-based detection of traffic. A proxy can limit or direct traffic based on more extensive factors than a network firewall can, but it's not capable of using the same signature detection rules as an IPS.