Packet Filtering Firewall

Packet filtering is a network security mechanism that works by controlling what data can flow in and out of the network. The basic device that interconnects IP networks is called a router. Packets that are traversing the network travel from router to router until they reach their destination. Each packet contains the address of its ultimate destination which tells the router where it wants to go. Routers communicate with each other through routing protocols like Routing Information Protocol (RIP)  and Open Shortest Path First (OSPF)  and build routing tables in their memories. These tables help the routers to determine how to get the packets to their destination. While routing a packet, a router compares the packet’s destination address to the entries in the routing table and forward the packets as directed by the routing table. In determining how to forward a packet towards it destination, a packet filtering router also analyzes each packet against a set of rules and those that conform to these rules are allowed to pass through while others are discarded.