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Your router essentially shares your Internet connection among multiple devices. A typical router is now a wireless router, and it creates and hosts a Wi-Fi network multiple devices can connect to. It likely has multiple Ethernet ports, too, allowing you to connect multiple devices.

The router then connects to the Internet through the modem and the router itself receives a single public IP address on the Internet. Servers on the Internet communicate with your router, and the router routes that traffic to the appropriate devices on your home network.

But, with just a router, you can’t actually connect to the Internet. The router must be plugged into the Internet via an Ethernet cable. You need a modem to do so.

What a Modem Does

Your modem communicates with your Internet service provider’s network. If it’s a cable modem, it plugs into your cable provider’s infrastructure via a coaxial cable. If it’s a DSL modem, it plugs into your telephone line.

The modem communicates with your Internet service provider, and you’ll need the correct type of modem that will work with your ISP’s infrastructure.

The modem plugs into whatever type of infrastructure you have — cable, telephone, satellite, or fiber — and gives you a standard Ethernet cable output that you can plug into any router (or a single computer) and get an Internet connection.

A modem (modulator-demodulator) is a device or program that enables a computer to transmit data over, for example, telephone or cable lines.
A device that forwards data packets along networks. A router is connected to at least two networks and are located at gateways.