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A perimeter security measure permitting only authorised LAN access to and from the Internet. Access can be administered according to job description and user requirements rather than a 'one rule suits all' basis.
The firewall analyses the traffic routed between the network and the Internet, both inbound and outbound, against set access criteria. Non-compliant traffic is stopped.
Firewalls fall into four categories: packet filters, circuit level gateways, application level gateways and stateful multilayer inspection firewalls.

  • Packet filtering firewall - Operating at the Network layer of the OSI analysing each packet is against a pre-determined set of criteria before being forwarded. Once analysed the firewall can drop the packet, forward it, or send a message to the originator.
  • Circuit level gateways - Operating at the Session layer of the OSI. TCP handshaking between packets is analysed to determine the legitimacy of requested sessions. Packets are not filtered by circuit level gateways.
  • Application level gateways (proxies) - Application specific, filtering packets at the application layer of the OSI model. An application level gateway that is configured to be a web proxy will not allow any ftp, gopher, telnet or other traffic through. User activity and logins and also be logged.
  • Stateful inspection firewall - A combination the other above firewalls. Packets are filtered at the network layer to determine whether session packets are legitimate and contents of packets are analysed at the Application layer. Algorithms identify and process application layer data as opposed to application specific proxies.
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