A modem connects your home to your internet provider. A router takes that one connection and shares it with all your devices, over Wi-Fi and Ethernet. Many providers hand out a single box, called a gateway, that does both jobs. When your speed is slow, the router's Wi-Fi is far more often the culprit than the modem.
Think of it this way: the modem is the front door to the internet, and the router is the hallway that leads to every room.
What a modem does
A modem translates the signal on your provider's line into a standard Ethernet connection that computers understand. What kind of modem you need depends on your connection:
- Cable internet uses a cable modem on the coaxial line, built to a standard called DOCSIS.
- DSL (including UK "part fibre", or FTTC) uses a DSL modem plugged into the phone socket.
- Fiber uses an optical network terminal (ONT), a small box that converts light in the fiber to Ethernet. When Openreach installs full fibre in the UK, for example, an engineer fits an ONT where the fiber enters the home.
- 5G and fixed wireless use a receiver with a SIM or antenna, usually combined with a router.
An ONT isn't always called a modem, but it plays the same role: it's the box that speaks your provider's language. Types of internet connection explains each connection type in more detail.
A modem on its own gives you one connection, for one device. That's where the router comes in.
What a router does
A router creates your home network. It:
- Shares the connection between every phone, laptop, TV and console.
- Broadcasts Wi-Fi and provides Ethernet ports for wired devices.
- Hands out local addresses to each device and keeps track of which traffic belongs to whom.
- Protects the network with a firewall between your devices and the internet.
Mesh systems are routers too, just spread across several units so the Wi-Fi reaches further.
Modem, router or gateway?
There are two common setups:
| Separate modem and router | Gateway (combo unit) | |
|---|---|---|
| Boxes | Two | One |
| Upgrade Wi-Fi | Replace just the router | Replace the whole unit, or add your own router |
| Setup | A little more work | Plug in and go |
| Placement | Router can go somewhere central | Stuck wherever the line comes in |
Gateways are convenient, but placement is the catch. If the line enters your home in a corner room, a gateway's Wi-Fi starts from that corner. With a separate router, you can run an Ethernet cable from the modem and put the router somewhere central.
If you add your own router to an ISP gateway, set the gateway to bridge mode (sometimes called modem mode) if it offers one. Otherwise you end up with two routers in a row, which can cause problems with gaming, port forwarding and some smart-home devices.
Which box is slowing you down?
Run two speed tests to find out:
- Plug a computer directly into the router (or gateway) with an Ethernet cable and run the SpeedMetric speed test.
- Run the same test on Wi-Fi in the room where you have trouble.
If the wired result is close to your plan but Wi-Fi is much slower, the router's Wi-Fi is the bottleneck, or the distance and walls between you and it. Why Wi-Fi is slower than Ethernet covers the usual causes.
If the wired result is also far below your plan, the problem is upstream: the modem, the line or your provider. One more thing to check is the Ethernet port speed on each box. A port rated at 1 Gbps can never pass more than 1,000 Mbps, so a 2 Gbps plan needs a modem and router with faster ports to reach a single device at full speed.
Signs that point at each box:
- Modem or line: the modem's status lights show a lost connection, every device drops at once, or wired speeds are low too.
- Router: Wi-Fi is weak in parts of the home, it slows down when many devices connect, or a restart makes it fast again for a while. Signs your router is slowing your internet has the full list.
If the internet drops entirely while your devices still show Wi-Fi bars, the router is working but the modem has lost its link. Restart the modem first, then check your provider's outage page.
When to replace each
Replace the router when:
- It can't cover your home, even after you've moved it somewhere central.
- Your Wi-Fi tests near it fall well short of wired tests on a fast plan.
- It no longer gets security updates from its maker.
A newer Wi-Fi generation helps only if your devices support it. Wi-Fi 6 vs Wi-Fi 7 explains when that's worth it, and how to choose a router covers what to look for.
Replace the modem when:
- You upgrade to a plan faster than the modem supports. Your provider can tell you which models handle your speed.
- It drops the connection often and your provider has ruled out the line.
- You're renting one and would rather buy a compatible model to stop the monthly fee.
For fiber, the ONT usually belongs to the provider and isn't something you replace yourself. If you suspect it's faulty, contact them.
Common questions
Can I use a router without a modem?
Not for internet access. The router needs something to translate your provider's line, whether that's a modem, an ONT or a gateway with one built in. Without it, the router only creates a local network.
Can I use my own modem instead of the ISP's?
Often, yes, but it has to be compatible with your provider's network, and some providers need to register it before it works. Check your provider's list of approved devices first.