What a modem does

A modem (short for modulator-demodulator) converts the signal on your ISP's line into standard Ethernet data your equipment understands, and back again. Which kind you have depends on how internet reaches your home:

  • Cable modem (DOCSIS): used by Xfinity, Spectrum, Cox, Optimum and other cable ISPs over coaxial cable. DOCSIS 3.1 is the current mainstream standard for gigabit-class plans; DOCSIS 4.0 is rolling out in some areas.
  • Fiber ONT (optical network terminal): converts light on the fiber into Ethernet. Used by Verizon Fios, AT&T Fiber, Google Fiber, Frontier Fiber and others.
  • DSL modem: uses copper phone lines. Still found in areas served by older telephone-company networks.
  • 5G/LTE gateway: a cellular modem built into the gateway, used by T-Mobile Home Internet, Verizon 5G Home and similar services.

A standalone modem doesn't create Wi-Fi or a home network. It usually has one Ethernet port and hands the connection to whatever is plugged into it, normally a router.

What a router does

The router takes the single connection from the modem and builds your home network around it:

  • NAT: lets all your devices share one public IP address.
  • DHCP: hands out local addresses such as 192.168.1.x or 10.0.0.x to devices that join.
  • Wi-Fi: broadcasts your wireless networks on 2.4, 5 and sometimes 6 GHz.
  • Ethernet switching: LAN ports for wired devices.
  • Firewall: blocks unsolicited inbound connections by default.
  • Extras: guest networks, parental controls, port forwarding, DNS settings, VPN, QoS.

The router is what you log in to at addresses like 192.168.0.1, 192.168.1.1 or 10.0.0.1. See how to log in to your router.

Gateways: the two-in-one box

Most US ISPs rent out a combined device, usually called a gateway. Inside are a modem and a router, managed together through one admin page or app. Typical examples:

  • Xfinity xFi Gateway: DOCSIS cable modem plus Wi-Fi router, managed largely through the Xfinity app.
  • Cox Panoramic Wifi gateway: cable modem plus router, managed through the Cox app.
  • AT&T fiber gateways: router with fiber connectivity; recent models include a built-in ONT.
  • T-Mobile and Verizon 5G home gateways: cellular modem plus Wi-Fi router.

Gateways are convenient — one box, one power cord, support from the ISP — but usually offer fewer settings than a retail router and come with a monthly equipment fee unless it's included in your plan. The FCC's broadband consumer labels require providers to disclose equipment fees, which makes it easier to see what you're paying.

When you'll see separate devices

Fiber with a separate ONT

Verizon Fios commonly installs an ONT on a wall or in a garage or basement, with an Ethernet cable running to the router. Because the ONT does the modem job, you can replace the router with your own model if you prefer.

Customer-owned cable modem plus router

On cable, many people buy an approved DOCSIS modem (models such as the ARRIS SURFboard, NETGEAR Nighthawk and Motorola lines) and pair it with a router or mesh system of their choice. You get full control over the network and avoid a rental fee, at the cost of handling setup and troubleshooting yourself.

Gateway plus your own mesh or router

Very common: keep the ISP gateway and add a mesh system (eero, TP-Link Deco, NETGEAR Orbi, Google Nest Wifi) for better coverage. To avoid two competing networks and double NAT, either set the gateway to bridge mode / IP Passthrough or set the mesh to access point / bridge mode. See fixing weak Wi-Fi for when mesh makes sense.

How to tell what you have

  1. Count the boxes. One box with Wi-Fi = gateway. Two boxes = modem/ONT plus router.
  2. Check the incoming line. Coax screw connector = cable modem or gateway. Green-tipped fiber connector or a box on the wall = ONT. Phone jack = DSL.
  3. Count Ethernet ports. A modem usually has one. A router or gateway typically has a WAN port plus several LAN ports.
  4. Look for Wi-Fi details on the label. A network name and Wi-Fi password printed on the sticker means the device is a router or gateway.
  5. Search the model number on the label. The manufacturer's spec sheet will say "cable modem", "gateway", "router" or "ONT".

Should you use your own equipment?

  • Own modem makes sense on cable if you plan to keep the service for a while, your ISP allows it, and you're comfortable activating it.
  • Own router or mesh makes sense if coverage is poor, you want more control (DNS, VPN, parental controls), or you want Wi-Fi 6E/7 on your schedule rather than the ISP's. See best routers for 2026.
  • Stick with the gateway if your coverage is fine, the equipment fee is bundled, or your service (5G home internet, some voice bundles) requires it.

Frequently asked questions

Is the box my ISP gave me a modem or a router?

Most likely both. Cable, DSL and many fiber providers rent out a gateway: a single box with a modem and a Wi-Fi router inside. Xfinity's xFi Gateway, Cox's Panoramic gateway and AT&T's fiber gateways are examples. If the box has Wi-Fi and several Ethernet ports for your devices, it's acting as a router. If it has one coax or phone jack and a single Ethernet port, and no Wi-Fi, it's a standalone modem. Spectrum, for example, has often supplied a separate modem and router.

Do I need both a modem and a router?

You need both functions, but not necessarily two boxes. The modem function connects to your ISP's network; the router function shares that connection with your devices and creates Wi-Fi. A gateway does both. On fiber services where the ISP installs an ONT (Verizon Fios, for example), the ONT plays the modem role and you connect a router to it with Ethernet. On 5G home internet, the gateway includes the cellular modem and router together.

Can I buy my own modem instead of renting one?

On cable internet, usually yes: Xfinity, Spectrum and Cox publish lists of approved customer-owned DOCSIS modems, and owning one avoids the monthly equipment fee. Check the list before you buy and make sure the modem supports your plan's speed (DOCSIS 3.1 for gigabit-class plans). Some services, such as certain voice or bundled features, require the ISP's gateway. On fiber, you generally keep the ISP's ONT or gateway but can add your own router. On 5G home internet, you must use the provider's gateway.

What is bridge mode?

Bridge mode turns off the router part of a gateway so it behaves like a plain modem and passes the public IP to your own router. This avoids double NAT, which can cause Strict NAT types and break port forwarding. Cable gateways usually offer bridge mode in their settings or app. AT&T fiber gateways don't have a true bridge mode; they use IP Passthrough, which achieves a similar result. When you enable either, the gateway's Wi-Fi typically stops working, so your own router takes over.