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A '''firewall''' is a system which filters network and permits presumably safe [[internet]]/intranet traffic while blocking suspicious or unwanted communication.<ref>https://en.oxforddictionaries.com/definition/firewall</ref> There are several types of firewall and methods of filtering.

A '''firewall''' is a system which filters network activity and permits presumably safe [[internet]]/intranet traffic while blocking suspicious or unwanted communication.<ref>https://en.oxforddictionaries.com/definition/firewall</ref> There are several types of firewall and methods of filtering.

 

==Physical and Virtual==

==Physical and Virtual==

Firewalls can be either physical devices which sit on the network between the Internet and the internal client, or software systems which are installed onto the client's device.

Firewalls can be either physical devices which sit on the network between the Internet and the internal client, or software systems which are installed onto the client's device.

===Hardware===

===Hardware===

Hardware firewalls are generally considered the best front-line of defense. When an incoming connection reaches the protected network, the connection must first go through this hardware firewall.  If the connection is unwanted, it will generally be ignored.  In this case, the entity attempting to connect will simply have their request time out.  Some firewalls will outright reject the request, however, which is a less effective defense method. (If rejected outright, a potential attacker will know he has found an active device, whereas if the request is ignored, it will be more difficult to know if there is even a device to connect to.)<br />

Hardware firewalls are generally considered the best front-line of defense. When an incoming connection reaches the protected network, the connection must first go through this hardware firewall.  If the connection is unwanted, it will generally be ignored.  In this case, the entity attempting to connect will simply have their request time out.  Some firewalls will outright reject the request, however, which is a less effective defense method. (If rejected outright, a potential attacker will know he has found an active device, whereas if the request is ignored, it will be more difficult to know if there is even a device to connect to.)<br />

To the outside world, there is only one device (the firewall) on the protected network. Even when a connection is permitted, the communication still passes through the firewall using a specified port.  Therefore, even active connections do not reveal the internal [[IP address]] of devices.<ref name="WhatIsMyIPAddress.com">“What is a Firewall?” WhatIsMyIPAddress.com, whatismyipaddress.com/firewall. Accessed 7 Sept. 2017.</ref><br />

To the outside world, there is only one device (the firewall) on the protected network. Even when a connection is permitted, the communication still passes through the firewall using a specified port.  Therefore, even active connections do not reveal the internal [[IP address]] of devices.<ref name="WhatIsMyIPAddress.com">“What is a Firewall?” WhatIsMyIPAddress.com, whatismyipaddress.com/firewall. Accessed 7 Sept. 2017.</ref><br />

However, many hardware firewalls (all but the newest types) often do not know what connections are being used for.  Most do not collect the packets and reassemble messages, so connections can be abused without the firewall knowing.  Even the ones which do attempt to analyze traffic can have difficulty understanding what is going on.<ref name="WhatIsMyIPAddress.com">“What is a Firewall?” WhatIsMyIPAddress.com, whatismyipaddress.com/firewall. Accessed 7 Sept. 2017.</ref>

However, many hardware firewalls (all but the newest types) often do not know what connections are being used for.  Most do not collect the packets and reassemble messages, so connections can be abused without the firewall knowing.  Even the ones which do attempt to analyze traffic can have difficulty understanding what is going on.<ref name="WhatIsMyIPAddress.com">“What is a Firewall?” WhatIsMyIPAddress.com, whatismyipaddress.com/firewall. Accessed 7 Sept. 2017.</ref>

===Software===

===Software===

Software firewalls are not typically considered a good first line of defense, but a good addition to a tiered defense system.  They digitally intercept and attempt to analyze traffic on the client's system. Since this kind of firewall operates on the [[OSI model]]'s application layer, it has an easier time of understanding what the communication link is being used for.<br />

Software firewalls are not typically considered a good first line of defense, but a good addition to a tiered defense system.  They digitally intercept and attempt to analyze traffic on the client's system. Since this kind of firewall operates on the [[OSI model]]'s application layer, it has an easier time of understanding what the communication link is being used for.<br />

===Unified threat management===

===Unified threat management===

These firewalls use stateful inspection, but also offer some additional functionality.  They incorporate some kind of intrusion prevention system to help prevent network penetration by attackers.  They also include an [[antivirus]] solution to help keep [[malware]] out of the network.

These firewalls use stateful inspection, but also offer some additional functionality.  They incorporate some kind of intrusion prevention system to help prevent network penetration by attackers.  They also include an [[antivirus]] solution to help keep [[malware]] out of the network.<ref name="Cisco">“What Is a Firewall?” Cisco, 6 Sept. 2017, www.cisco.com/c/en/us/products/security/firewalls/what-is-a-firewall.html. Accessed 7 Sept. 2017.</ref>