Which device is primarily used to connect multiple networks together?

The device used primarily to connect multiple networks together is a router. Routers operate by forwarding data packets between different networks, enabling communication between devices on separate local area networks (LANs) or between a LAN and a wide area network (WAN). They perform this function by using routing tables and protocols to determine the best path for data to travel, ensuring efficient data transfer.

In contrast, while switches connect multiple devices within the same network segment and facilitate communication among them, they do not serve to connect different networks. Access points provide wireless connectivity to devices within a specific network but do not handle routing between networks. Firewalls, on the other hand, are security devices that monitor and control incoming and outgoing network traffic based on predetermined security rules; they can be involved in routing decisions but are not designed to directly connect networks in the way that a router does. Thus, a router is the correct answer, as it fulfills the role of interlinking distinct networks effectively.