What are packet filters and how do they work?
Packet filters are installed on a customer’s gateway router to specify what packets the router should pass and which packets the router should refuse to pass. Filters can be written based on the source and destination address and port numbers. This allows decisions based not only on the machines involved but the protocols as well. As an example, it is possible to allow telnet connection to only one host on a customer LAN but allow mail connections to any hosts.
