What is Firewall (networking)?
{{redirect|Firewall}} [[File:Gateway firewall.svg|thumb|A Firewall protects a number of computers on a [[Local Area Network|LAN]] against unauthorised access.]] In terms of [[computer security]], a '''firewall''' is a piece of [[software]]. This software monitors the network traffic between the inside and outside. The firewall is placed between the network that is to be protected (trusted) and the outside network (less trusted) aka [[Wide area network|WAN]] or [[Internet]]. A firewall has a set of [[rules]] which are applied to each [[packet (computing)|packet]]. The rules decide if a packet can pass, or whether it is discarded. When a large network needs to be protected, the firewall software often runs on a computer that does nothing else. A firewall protects one part of the network against unauthorized access. ==Different kinds of Firewalls == ===Packet filtering / Network Layer === Data travels on the internet in small pieces; these are called [[Packet (information technology)|packet]]s. Each packet has certain [[metadata]] attached, like where it is coming from, and where it should be sent to. The easiest thing to do is to look at the metadata. Based on rules, certain packets are then dropped or rejected. It is done at the network layer. ===Stateful packet inspection=== In addition to the simple packet filtering (above), this kind of firewall also keeps track of connections. A packet can be the start of a new connection, or it can be part of an existing connection. If it is neither of the two, it is probably useless and can be dropped. ===Application-layer firewalls=== Application-layer firewalls do not just look at the metadata; they also look at the actual data transported. They know how certain [[protocol]]s work, for example [[FTP]] or [[HTTP]]. They can then look if the data that is in the packet is valid (for that protocol). If it is not, it can be dropped. ==Other things firewalls are used for== ===Tunneling=== Firewalls can provide a secure connection between two networks. This is called ''tu