Which device is primarily used to protect computer networks from unwanted traffic?

The primary purpose of a firewall is to protect computer networks from unwanted traffic by monitoring and controlling incoming and outgoing network traffic based on predetermined security rules. This device acts as a barrier between a trusted internal network and untrusted external networks, preventing unauthorized access and potential threats from reaching sensitive data or systems within the network.

Firewalls can be implemented in both hardware and software forms and are essential for establishing a network's security perimeter. They analyze data packets and decide whether to allow or block them, effectively filtering harmful traffic while permitting legitimate communications to take place. This capability is crucial for maintaining the integrity, confidentiality, and availability of networked systems.

In contrast, a router directs traffic between different networks, a switch connects devices within a single network, and a modem serves as a bridge between a computer network and the internet. While these devices play important roles in network infrastructure, their functions do not specifically focus on filtering or blocking unwanted traffic like that of a firewall.