|  |  | An Intrusion Detection System (IDS) monitors every packet traversing your network as if it were a protocol analyzer, and then makes a security decision based on that packet. One packet may not look suspicious by itself, but if you combine it with one that passes through the IDS eighty packets later, you may be able to recognize an attack in progress. An IDS can make its decisions based on several packets, instead of just one like the typical firewall.