Glossary · Networking

Modem

A modem is a device that converts digital data into the signal type of a physical line, such as cable, DSL or fiber, and back again.

Updated October 6, 2026

A modem (modulator-demodulator) translates between your network's digital data and the analog or optical signal used on your provider's line. It connects you to the access network but does not decide how traffic is routed on your LAN.

Why it matters

Your Internet service enters the building through a cable, telephone, or fiber line, and the modem or optical terminal makes that line usable by Ethernet devices. Many providers ship a combined modem and router (a gateway). Knowing which function is failing helps you diagnose an outage: no sync light points to the line or provider, while a working link without Internet points to configuration.

Example

A cable setup typically looks like this:

Coax wall jack -> modem -> router -> switch/Wi-Fi -> devices

If the modem's online light is solid but devices cannot browse, test the router. If the modem keeps resyncing, contact the provider.

Common mistakes

  • Calling the whole provider box a modem when it also contains a router and Wi-Fi.
  • Putting a second router behind a gateway without realizing it creates double NAT.
  • Skipping a power cycle of modem and router in the correct order during basic troubleshooting.

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This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.