A router and a modem may be combined into one device by some Internet service providers (ISPs), but they are not the same. Each participates in a unique but equally crucial function in tying networks to the Internet and one another.
While a modem links those networks to the Internet, a router establishes the networks and controls the data flow inside and among them. By transforming signals from an ISP into digital signals that can be understood by any connected device, modems create a connection to the Internet. To connect to the Internet, a single device can plug into a modem; alternatively, a router can help spread this signal among numerous devices connected to an established network, enabling all of them to connect to the Internet at once.
Consider this: If Bob has a router but no modem, he can still set up a local area network (LAN) and transport data among the connected devices. He won’t be able to link that network to the Internet though. Alice, on the other hand, only has a modem. She’ll be able to use one device to access the Internet (like her laptop for work), but she won’t be able to share that Internet connection with several devices (say, her laptop and her smartphone). Carol, on the other hand, has a modem and a router. She can connect her desktop computer, tablet, and smartphone to the Internet simultaneously by creating a LAN using both of her devices.