IP-based computer networks are a key component of modern communication infrastructures. As different types of participants, such as corporations, public officials, and individuals, rely on complex and sophisticated services and communication systems, the deployment of these networks is accelerating at an exponential pace. This presents new challenges in information security because large amounts of data that could contain malicious content, such as viruses, worms, or Trojans, can be transferred over open networks. These threats can be addressed by network security measures that are implemented both within the network and at hosts connected to access routers. The host-based approach has its advantages, especially in terms of the scalability and security framework. For example, placing security capabilities like firewalls or virus scanners on individual hosts doesn't hinder traffic through the network.