Firewalls generally use two or more of the following methods:

• Packet Filtering: Firewalls filter packets that attempt to enter or leave a network and either accept or reject them depending on the predefined set of filter rules.
• Application Gateway: The application gateway technique employs security methods applied to certain applications such as Telnet and File Transfer Protocol servers.
• Circuit-Level Gateway: A circuit-level gateway applies these methods when a connection such as Transmission Control Protocol is established and packets start to move.
• Proxy Servers: Proxy servers can mask real network addresses and intercept every message that enters or leaves a network.
• Stateful Inspection or Dynamic Packet Filtering: This method compares not just the header information, but also a packet’s most important inbound and outbound data parts. These are then compared to a trusted information database for characteristic matches. This determines whether the information is authorized to cross the firewall into the network.