Defining Firewalls
Firewalls are usually a combination of hardware and software. The hardware part is usually a router, but it can also be a computer or a dedicated piece of hardware called a black box that has two network interface cards (NICs) in it. One of the NICs connects to the public side, and the other one connects to the private side. The software part is configured to control how the firewall actually works to protect your network by scrutinizing each incoming and outgoing packet and rejecting any suspicious ones.
Firewalls generally allow only packets that pass specific security restrictions to get through; they can also permit, deny, encrypt, decrypt, and proxy all traffic that flows through, either between the public and private parts of a network or between different security domains, or zones, on a private network. The system administrator decides on and sets up the rules a firewall follows when deciding to forward data packets or reject them.
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