Firewalls provide a variety of services to networks in terms of security. They provide for network address translation (NAT), virtual private networks (VPN), and filtering of traffic that does not conform to the network's stated security policy. There are many forms of firewalls from simple packet filters to circuit-level gateways to proxy firewalls. Firewalls are being asked to fill a larger and more varied role in network security these days than several years ago. One of the more recent innovations in firewall technology is the application of deep packet inspection or DPI. Deep Packet Inspection can be seen as the integration of Intrusion Detection (IDS) and Intrusion Prevention (IPS) capabilities with traditional stateful firewall technology. Traditional networks have a defined boundary demarcated by a firewall with an IDS sensor sitting behind it.

One of the primary benefits of the traditional firewall/IDS deployment is that the failure of one component does not leave the network completely unprotected. Also, IDS appliances can be deployed throughout the LAN and monitor traffic inside the LAN as opposed to boundary areas between networks. This design is illustrated in Figure 1 below. The IDS monitors traffic that passes through the firewall (as defined in the firewall policy) and inspects packets for malicious activity.

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