What is a firewall ? Briefly explain different firewall techniques.

The system designed to prevent unauthorized access to or from a private network is called Firewall.

There are several types of firewall techniques :

1. Packet filter — It 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.
2. Application gateway — It applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose a performance degradation.
3. Proxy server — It intercepts all messages entering and leaving the network. The proxy server effectively hides the true network addresses.
4. Circuit-level gateway — It applies security mechanisms when a connection is established. Once the connection has been made, packets can flow between the hosts without further checking.