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Firewall is a piece of software or hardware that protects your computer and the information on it from attackers trying to access it. It does this by acting as a 24/7 filter, examining every piece of data that attempts to enter or leave your computer and blocking anything that looks suspicious.

Firewalls can be either hardware or software, with many different types. Most commonly, a firewall is installed on a host computer to limit the activity it accepts from the internet and within its private network. This can include limiting what applications can access the internet, determining what programs are allowed to run, and managing protocol and port numbers.

There are also network-based firewalls that manage the entire network perimeter, ensuring all hosts in the network are secure. These are often used in larger businesses and can be more complex to configure and operate.

Another type of firewall is the stateful inspection architecture, which works more like a guard at a doorway. It keeps a history of the traffic it sees and makes decisions on whether to allow or deny each packet of data based on where it came from, where it’s going, and if it’s a new connection. If a packet is denied, it’s turned away from the gate and sent back to where it came from, while a dropped packet just ceases to exist.

Finally, there are proxy-based firewalls that can provide application layer protection. This is the closest to an actual physical barrier and allows deeper filtering of incoming packets based on more than just IP addresses, ports and basic packet protocols (UDP and ICMP) as packet-filtering and circuit-level gateway firewalls can do.