What device scans and blocks traffic entering or leaving a network?

A firewall is a security device that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It serves as a barrier between a trusted internal network and untrusted external networks, such as the Internet. Firewalls can be hardware-based, software-based, or a combination of both and are essential for protecting networks from unauthorized access, threats, and attacks. By analyzing the traffic flowing through it, the firewall can effectively block or allow data packets according to the security policies in place, helping to maintain the integrity and security of the network.

The other options serve different functions: a modem connects to the internet and converts digital data to analog for transmission over telephone lines, a hub connects multiple devices in a network but does not filter traffic, and a switch connects devices but operates primarily at a data link layer, forwarding data within the network without considering security measures.