A firewall is a device or set of devices designed to permit or deny network transmissions based upon a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass.

First generation: packet filters

 The first paper published on firewall technology was in 1988, when engineers from Digital Equipment Corporation (DEC) developed packet filter firewalls.This type of packet filtering pays no attention to whether a packet is part of an existing stream of traffic. Instead, it filters each packet based only on information contained in the packet itself (most commonly using a combination of the packet’s source and destination address, its protocol, and, for TCP and UDP traffic, the port number).

 Second generation: application layer

 The key benefit of application layer filtering is that it can “understand” certain applications and protocols (such as File Transfer Protocol, DNS, or web browsing), and it can detect if an unwanted protocol is sneaking through on a non-standard port or if a protocol is being abused in any harmful way.An application firewall is much more secure and reliable compared to packet filter firewalls because it works on all seven layers of the OSI model, from the application down to the physical Layer. This is similar to a packet filter firewall but here we can also filter information on the basis of content.

 Good examples of application firewalls are MS-ISA (Internet Security and Acceleration) server, McAfee Firewall Enterprise & Palo Alto PS Series firewalls.

 Third generation: “stateful” filters

 From 1989-1990 three colleagues from AT&T Bell Laboratories, Dave Presetto, Janardan Sharma, and Kshitij Nigam, developed the third generation of firewalls, calling them circuit level firewalls.

 Third-generation firewalls, in addition to what first- and second-generation look for, regard placement of each individual packet within the packet series. This technology is generally referred to as a stateful packet inspection as it maintains records of all connections passing through the firewall and is able to determine whether a packet is the start of a new connection, a part of an existing connection, or is an invalid packet.

 In 1994 an Israeli company called Checkpoint Software Technologies built visual integration interface with colors and icons into readily available software known as FireWall-1.