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A firewall is a hardware device or software application that shields your computer network from attacks by monitoring the data that goes in and out of your system. Firewalls are constantly on guard, looking at the millions of packets that try to enter your system and weeding out those that look suspicious. Think of it like a bouncer at a nightclub that knows the rules to keep out people who don’t belong there.

A basic firewall is inserted inline across the connection between an external network and your protected system. It monitors all of the traffic, or data packets, that attempt to pass through the firewall, and compares them against a set of preconfigured rules. If it sees a match, it blocks the packet and sends it back to the outside network. If it can’t find a match, it passes the packet on its way.

Each data packet has a unique address, called a port number, that designates where it originated from and where it is going. It also has a unique ID that designates the type of information it contains (like an email, a file or a webpage). A firewall scans each data packet to determine its content and whether it is attempting to gain access to your system. Based on its findings, it may accept, deny or drop the packet.

Depending on the configuration, a firewall may offer additional security functionalities beyond access control. For example, some can help prevent network outages by rerouting spoofed packets away from affected systems and support virtual private networks to enhance privacy and security in remote connections. Keeping the threat landscape in mind, it is important to update firewalls frequently and carefully so as not to leave a door open for future attackers.