What is the primary purpose of a firewall in network security?

The primary purpose of a firewall in network security is to monitor and control incoming and outgoing network traffic. Firewalls act as a barrier between trusted internal networks and untrusted external networks, such as the internet. By assessing and filtering the traffic based on predetermined security rules, firewalls help protect systems from unauthorized access and various types of cyber threats.

Firewalls can be configured to allow or block specific traffic based on criteria such as IP addresses, domain names, protocols, and ports. This capability is crucial for maintaining the integrity of a network and ensuring that only legitimate traffic is permitted, thereby reducing the risk of attacks, malware infections, and data breaches. They can operate at different levels of the network stack, including packet filtering, stateful inspection, and application-layer firewalls, each providing varying degrees of control and security.

The other options do not encompass the main function of a firewall. Encrypting sensitive data relates more to data security during transmission, while user authentication focuses on verifying the identity of users rather than controlling network traffic. Physical security pertains to the safeguarding of hardware and infrastructure, which is outside the realm of what a firewall is designed to do.