What program is designed to ensure that only authorized users can access a network's intranet?

A firewall is a program or device designed to protect networks from unauthorized access. It acts as a barrier between an internal network (intranet) and external networks (like the internet), controlling the flow of incoming and outgoing traffic based on predetermined security rules. By inspecting packets of data and determining their legitimacy, firewalls can block unauthorized users from accessing sensitive data and resources within the network.

In contrast, a router is primarily responsible for directing data packets between different networks. A switch connects devices within the same network and manages communication between them. A modem serves to convert digital data from a computer to an analog signal for transmission over telephone lines or vice versa. While these devices play essential roles in network functionality, they do not specifically focus on ensuring that access to the intranet is limited to authorized users. Thus, the firewall is specifically designed to address security and access control within a network.