You'll also need a virus scanner to block viruses that contain Trojan Horses. Trojan Horses are intruder applications that cooperate with their creators to do something you probably wouldn't like. Many speculate that a large number of Trojan Horses were used in some of the recent DDoS attacks. Such viruses listen on an unused TCP or UDP port and, when signaled on that port, flood a site with traffic. A firewall can be either a software product installed on each system or a hardware firewall that sits between the LAN and the cable modem or DSL adapter.
Many users think they're too small to be bothered by hackers.
Several hackers attempted to open FTP sessions to read or write files, and several attempted to plant Trojan Horses. A good security product should have special scanning rules that warn you if somebody tries to plant a known Trojan Horse. It should also block any unused ports and log outside attempts to contact ports. The Norton product logs any outside attempts to contact an unused port, and it's possible to run the IP address of the source through an IP lookup facility to see if you can retrieve the domain name. In my case, running the offending addresses backward to decode the source revealed that half the addresses originated from other cable modem and DSL users, while the other half didn't decode at all.