A simplified definition for a network firewall is that it is a tool that implements the traffic checking that has to be done (for security purposes) among two or more networks.

The network firewall has multiple missions; primarily, the firewall is in charge of monitoring, inspecting, and controlling inbound/outbound network traffic. The network firewall utilizes the user-defined preferences in order to determine whether to permit or deny communication with a specific network. Those user-defined preferences usually define the characteristics and the criteria by which the firewall determines whether to allow or discard the network traffic. Specifically, packets will be checked to examine their purposes. If the content represents hostile intrusion attempts, unauthorized attempts or patterns that match those of denial of service attacks, then the firewall will automatically block them.
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