Beginning of Essay
The term firewall has seen limited use since the late 1980s to describe a device to block unwanted network traffic while allowing other traffic to pass. The first published description of a "modern" firewall including use of that name was in 'Practical Unix Securi ....

Middle of Essay
.... varies widely, but in principle, the firewall can be thought of as a pair of mechanisms: one that exists to block traffic, and the other that exists to permit traffic. Some place a greater emphasis on blocking traffic, while others emphasize permitting traffic. Probably the most important thing to recognize about a firewall is that it implements an access control policy. The Internet, like any other society, is plagued with the kind of jerks who enjoy the electronic equivalent of writing on other people's walls with spray paint, tearing their mailboxes off, or just sitting in the street blowing their car horns. Some people try to get real work don ....