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→Packet-Filtering Firewalls
== Packet-Filtering Firewalls ==
Packet-filtering firewalls operate at the Network layer (layer 3) of OSI model are the most basic of firewall types. The concept of packet filtering involves defining which data packets are permitted to pass through the firewall based on a number of criteria. Essentially, filtering can be specified based IP addresses, ports and protocols. It is possible, for example to block any packets originating from a particular IP address (or IP address range), or to block all traffic attempting to enter on port 23 (the Telnet port).
Packet-filtering firewalls are typically built into routers and provide either a command-line to graphical interface for specifying the filtering rules. The defenses provided by such firewalls are considered to be weak since they can only block what IP addresses that an administrator knows in advance are likely to be malicious. They are, however, extremely fast and easy to configure, and provide good first line of defense.
== Proxy Service Firewalls ==