A firewall is a device (usually a router or a computer) installed between the internal network of an organization and the rest of the Internet. A firewall is designed to control the access to a system.
A firewall is classified as:
- Packet filter firewall
- Proxy-based firewall
A packet filter firewall filters at the network or transport layer.
A proxy firewall filters at the application layer. For example: an organization wants to implement the following policies regarding its Web pages: Only those Internet users who have previously established business relations with the company can have access; access to other users must be blocked. In this case proxy firewall is used.