Firewalls use one or more of three methods to control traffic flowing in and out of the network:
- Packet filtering - Packets (small chunks of data) are examined against a set of filters. Packets that make it by the filters are sent to the requesting system and all others are discarded.
- Proxy service - Information from the Internet is retrieved by the firewall and then sends to the requesting system and it's vice versa
- Stateful inspection - A newer process that doesn't analyse the contents of each packet but instead compares certain key parts of the packet to a database of trusted information.