What is the primary function of a firewall in a FortiGate device?

The primary function of a firewall in a FortiGate device is to inspect and filter incoming and outgoing traffic. This involves analyzing network packets against a set of predefined security rules to determine whether to allow or block the traffic based on various criteria, such as IP addresses, protocols, and application types.

Firewalls are critical components in network security as they create a barrier between trusted internal networks and untrusted external networks. By inspecting traffic, FortiGate firewalls can help prevent unauthorized access, mitigate threats, and enforce organizational security policies. This functionality enables the device to not only control traffic flow but also to enhance security by applying intrusion prevention, antivirus scanning, and other security measures.

The other options do not accurately represent the comprehensive role of a firewall. Allowing all traffic by default, blocking all traffic regardless of rules, or only permitting traffic from predefined IP addresses are too simplistic and do not capture the essential aspect of active inspection and filtering that defines a firewall's purpose.