A firewall provides a single point of defense between two networks—it protects one network from the other. Usually, a firewall protects the company's private network from the public or shared networks to which it is connected. A firewall can be as simple as a router that filters packets or as complex as a multi-computer, multi-router solution that combines packet filtering and application-level proxy services.
Firewall technology is a young but quickly maturing industry. The first generation of firewall architectures has been around almost as long as routers, first appearing around 1985 and coming out of Cisco's IOS software division. These firewalls are called packet filter firewalls. However, the first paper describing the screening process used by packet filter firewalls did not appear until 1988, when Jeff Mogul from Digital Equipment Corporation published his studies. During the 1989-1990 timeframe, Dave Presotto and Howard Trickey of AT&T Bell Laboratories pioneered the second generation of firewall architectures with their research in circuit relays, which are also known as circuit level firewalls. They also implemented the first working model of the third generation of firewall architectures, known as application layer firewalls.
However, they neither published any papers describing this architecture nor released a product based upon their work. As is often the case in research and development, the third generation of firewall architectures was independently researched and developed by several people across the United States during the late 1980's and early 1990's. Publications by Gene Spafford of Purdue University, Bill Cheswick of AT&T Bell Laboratories, and Marcus Ranum describing application layer firewalls first appeared during 1990 and 1991. Marcus Ranum's work received the most attention in 1991 and took the form of bastion hosts running proxy services. Ranum's work quickly evolved into the first commercial product—Digital Equipment Corporation's SEAL product.
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