Network firewalls are security devices used to stop or mitigate unauthorized access to private networks connected to the Internet, especially intranets. The only traffic allowed on the network is defined via firewall policies – any other traffic attempting to access the network is blocked. Network firewalls sit at the front line of a network, acting as a communications liaison between internal and external devices.
A network firewall can be configured so that any data entering or exiting the network has to pass through it – it accomplishes this by examining each incoming message and rejecting those that fail to meet the defined security criteria. When properly configured, a firewall allows users to access any of the resources they need while simultaneously keeping out unwanted users, hackers, viruses, worms or other malicious programs trying to access the protected network.
To know more: firewall concepts in network security