What does a firewall do?

A firewall functions primarily as a security device that monitors and controls network traffic based on predetermined security rules. Its main purpose is to establish a barrier between trusted internal networks and untrusted external networks, such as the internet. By doing so, it helps prevent unauthorized access to private networks while allowing legitimate traffic to pass through. Firewalls can filter traffic based on various criteria, such as IP addresses, port numbers, and protocols, thus enhancing the security of the network.

The other options describe different functionalities that are not related to the core purpose of a firewall. While improving internet connection speed might involve other types of network optimizations, encrypting sensitive data relates to securing information during transmission or storage, and facilitating remote access typically involves Virtual Private Networks (VPNs) or similar technologies.