A firewall is a set of related programs, located at a network gateway server, that protects the resources of a private network from users from other networks. An enterprise with an intranet that allows its workers access to the wider Internet installs a firewall to prevent outsiders from accessing its own private data resources. The firewall controls what outside resources its own users can have access to. Simply put, a firewall is a program or a hardware device that filters the information coming through the Internet connection into your private network or computer system. If an incoming packet of information is flagged by the filters it is not allowed through.

Firewalls use one or more of three methods to control traffic flowing in and out of the network:

• Packet filtering - Packets or small chunks of data are analyzed against a set of filters. Packets that make it through the filters are sent to the requesting system and all others are discarded.

• Proxy service - Information from the Internet is retrieved by the firewall and then sent to the requesting system and vice versa.

• Stateful inspection - A newer method that does not examine the contents of each packet but instead compares certain key parts of the packet to a database of trusted information. Information traveling from inside the firewall to outside is monitored for specific defining characteristics, then incoming information is compared to these characteristics. If the comparison yields a reasonable match the information is allowed through. Else, it is discarded.