In an effort to put the usage of these filters into context I will explain a normal day in the life of a network security analyst. This day will focus on the usage of building and further explaining some complex examples. To clarify our example, I assume that the make-believe network has all packets that are flagged by the intrusion detection system logged to a central database. I mention this stipulation because not every real network operates in this fashion. Some only turn on full logging after they've detected suspicious activity. For our purposes, though, the system logs all activity.

Our example network is a medium-sized corporate network that offers the following services to the outside world: HTTP, HTTPS, FTP, SMTP, and SSH. This is a small subset of possible services that a real network may offer. For example purposes, the people scanning our networks will have a 192.168/16 address and our network addresses will be within the 10.10/16 range.

By Don Parker at O'Reilly.

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