Control traffic between internal network & internet-a system designed to prevent unauthorized access to or from a private network. Here are several types of firewall techniques: # packet filter: looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules. Packet filtering is fairly effective and transparent to users, but it is difficult to configure. Inaddition, it is susceptible to ip spoofing. # application gateway: applies security mechanisms to specific applications, such as ftp and telnet servers. this is very effective, but can impose a performance degradation. # circuit-level gateway: applies security mechanisms when a tcp or udp connection is established. Once the connection has been made, packets can flow between the hosts without further checking. # proxy server: intercepts all messages entering and leaving the network. the proxy server effectively hides the true network addresses.
