Firewall

What is a firewall?
In network security, firewall is considered a first line of defense in protecting private information. "A firewall sits at the junction point, or gateway, between the two networks, which are often a private network and a public network such as the Internet." (John V. Harrison, Hal Berghel, 2004, A Protocol Layer Survey of Network Security). It is a system designed to prevent certain access to or from another network. It has two mechanisms and they are blocking or permitting accesses.
Why do I need a firewall?
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