A firewall is a hardware and/or software component or set of components that intercepts data and thereby restricts access between a protected network and outside networks such as the Internet. Firewalls are an essential ingredient in a corporate entity's network security plan. Firewalls represent a security enforcement point that separates a trusted network from an untrusted network. The most common use of a firewall is on a local area network (LAN) connected to the global information network, known as the Internet. A firewall provides services which enforce a boundary between two or more networks. A firewall typically comprises a static dedicated piece of code that operates by using a dedicated port. Each software component communicates with another component by knowing the dedicated port number of the other component. Firewalls have typically relied on packet filtering and proxy services for network protection. Packet filtering is the action a firewall takes to selectively control the flow of data to and from a network. A packet filter examines all incoming and outgoing data packets and, based on pre-defined filtering rules, determines which packets will be allowed to pass. Filtering rules can be based on one or more factors, including type of packet, source and destination IP address, port number, etc. A firewall may incorporate one or more proxies or proxy modules to handle particular functions or particular types of traffic received by the firewall. A proxy server operates at the upper levels of the protocol stack such as the application layer and presentation layer and provides proxy services on external networks for protected internal clients. The role of an application proxy is to communicate with external services on behalf of a client. While application proxies are more secure than packet filters, they are also much slower. A firewall is typically located at a point of entry into a computer system or network, such as a port or TCP/IP network interface, and scans incoming message traffic by comparing the message traffic to predetermined criteria.
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