How a firewall works

A firewall does all of the following tasks:
  • Prevents any unauthorized users from accessing the computers and networks in your organization that connect to the Internet
  • Monitors the communication between your computers and other computers on the Internet
  • Creates a shield that allows or blocks attempts to access the information on your computer
  • Warns you of connection attempts from other computers
  • Warns you of connection attempts by the applications on your computer that connect to other computers
The firewall reviews the packets of data that travel across the Internet. A packet is a discrete unit of data that is part of the information flow between two computers. Packets are reassembled at their destination to appear as an unbroken data stream.
Packets include the following information about the data:
  • The originating computer
  • The intended recipient or recipients
  • How the packet data is processed
  • Ports that receive the packets
    Ports are the channels that divide the stream of data that comes from the Internet. Applications that run on a computer listen to the ports. The applications accept the data that is sent to the ports.
Network attacks exploit weaknesses in vulnerable applications. Attackers use these weaknesses to send the packets that contain malicious programming code to ports. When vulnerable applications listen to the ports, the malicious code lets the attackers gain access to the computer.