What key function does a firewall serve within a network?

A firewall is primarily designed to protect a network by preventing unauthorized access to or from that network. It functions by monitoring and controlling incoming and outgoing network traffic based on predetermined security rules. This prevents unauthorized users and malicious entities from accessing sensitive information or systems within the network, thereby providing a critical layer of security.

In comparison, options relating to speeding up data transfer rates, enhancing cloud storage, or facilitating system upgrades do not align with the fundamental role of a firewall. Firewalls are not designed to optimize data flow or manage storage resources. Instead, their main purpose is to act as a barrier between trusted internal networks and potentially untrusted external networks, ensuring that any data that enters or leaves the network does so in a secure manner. This isolation and protection is what makes firewalls a vital component of network security strategies.