Final answer:
Firewalls are network security devices that keep internal networks secure while enabling communication with the rest of the Internet.
Step-by-step explanation:
The type of network security devices that keep internal networks secure while enabling communication with the rest of the Internet are firewalls. Firewalls act as a barrier between the internal network and the Internet, monitoring and controlling incoming and outgoing network traffic based on predetermined rules. They help prevent unauthorized access and protect against various types of cyber threats.
Routers, switches, and modems are important network devices, but they do not primarily focus on network security. Routers are responsible for directing traffic between different networks, while switches facilitate the communication between devices within a network. Modems enable the connection to the Internet, but they do not provide security functionalities like firewalls do.