In addition to protecting individual computers and servers attached to the network, it is important to control traffic traveling to and from the network.

A firewall is one of the most effective security tools available for protecting internal network users from external threats. A firewall resides between two or more networks and controls the traffic between them and also helps prevent unauthorized access. Firewall products use various techniques for determining what is permitted or denied access to a network. These techniques are:

Firewall products may support one or more of these filtering capabilities. Additionally, firewalls often perform Network Address Translation (NAT). NAT translates an internal IP address or group of IP addresses into an outside, public IP address that is sent across the network. This allows internal IP addresses to be concealed from outside users.

Firewall products come packaged in various forms, as shown in the figure.