You have a firewall at the perimeter, so your network is safe, right? Well, not exactly. Firewalls do what they do very well, but what they do is block or allow traffic based on the rules that you configure. A firewall can block certain ports that you don't need to use, preventing hackers from using those ports to get into the network. Firewalls can also block traffic based on IP address or domain, helping to keep out messages from known spammers or preventing Web traffic from known dangerous sites. Application layer filtering firewalls can even examine the contents of packets and block them based on keywords or strings.