Firewall Evolution - Deep Packet Inspection
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.
The link for this article located at SecurityFocus is no longer available.