In computer terminology, a firewall is a network security system that is entrusted with the task of monitoring and controlling inbound and outbound network traffic based on certain predetermined parameters. In other words, it is a network security device that grants or rejects network access to traffic flows between an untrusted zone (e.g., the Internet) and a trusted zone (e.g., a private or corporate network). Firewalls can be implemented in both hardware and software, or a combination of both.
Network firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet are scrutinised by the firewall, which determines the suitability of each message before allowing to them to pass through or restricting them. Firewalls enforce access controls based on positive control model, which states that only traffic defined in the firewall policy is allowed onto the network.