A firewall is a system that prevents unauthorized access to and from a network. On personal computers, the network protected is often a single computer. These systems typically rely upon a software firewall, that is, a computer program operating in the background that scrutinizes all incoming and outgoing data. Larger networks such as home LAN's, corporate networks, and universities utilize hardware firewalls. A hardware firewall is a standalone computer who's only purpose is to filter data entering and leaving the network. While both software and hardware firewalls have the same purpose, they operate differently and each has it's own advantages and disadvantages. In recent years, modem and router manufacturers have begun incorporating hardware firewalls into their products.
A firewall functions by examining packets of information passing through it and matching them against a set of rules. These rules may be either white- or black- lists of packet properties. Whitelists are tables of rules for known-good packets. For example, if a user wants all traffic going to work.com to be allowed, then she would whilelist the server at work.com and the firewall would permit all packets to and from work.com to pass. Whitelist systems block all Internet traffic for which there is no filter in place. Blacklists are just the opposite: they a tables of rules for known-bad packets. If a user knows that the server hacker.org had a history of attacking her computer, she can simply blacklist the server hacker.org. All packets coming from hacker.org will then be blocked from entering the network. The firewall can even be configured to alert the user in the unlikely event that her computer itself attempts to contact hacker.org, for instance, if the computer has a virus or spyware installed.
Although a dedicated hardware firewall can be anything from an inexpensive recycled Linux PC to a ten-thousand dollar Unix mainframe, most firewalls in home use today are software based. Companies such as McAfee, Comodo and Sygate offer free downloads of software based firewalls for the Windows operating system. Most of the free firewalls are in effect scaled-down versions of expensive commercial products that are limited to a certain amount of time that they can be used, or have features disabled. While Microsoft Windows does in fact ship with a built-in firewall, it is considered by most security experts as inadequate and was disabled by default in older versions of the OS. Other operating systems, such as Mac OS-X and Linux, have open source firewalls available that are often installed along with the OS.
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