Router or modem? What the box in your hallway really does

Two jobs, often in one box

Getting the internet into your home needs two separate jobs done:

• Talking to your internet provider over the line that comes into your home.
• Sharing that connection between all your phones, laptops, TVs and other gadgets.

The first job belongs to the modem. The second belongs to the router. In many homes one box does both, which is why the names get mixed up. Your provider might call it a hub, a gateway or simply "the router".

The modem: a translator at the door

The line coming into your home carries information in a form that suits that line: light pulses in a fibre cable, electrical signals on an old copper phone line, or a signal on a TV-style cable. Your computer does not understand any of these directly.

The modem is a translator. It turns the signal on the line into ordinary computer data, and back again for anything you send. The name comes from modulator-demodulator, which simply describes this converting back and forth.

With full fibre, the translating box is often a separate small unit on the wall where the fibre enters, called an ONT (optical network terminal). It does the modem's job for a fibre line.

A modem on its own usually connects just one device. That is where the router comes in.

The router: a post room for your home

A router takes that one connection and shares it. Think of the post room in a block of flats. Mail arrives for the building, and the post room sorts it to the right flat. Outgoing mail is collected and sent out together.

Your router does several things:

• Gives each device a private address inside your home, so it knows who asked for what. See What's an IP address?
• Directs traffic, so the film your child is watching goes to the TV, not to your laptop.
• Creates your Wi-Fi, the wireless network your devices join. See How Wi-Fi works.
• Acts as a basic barrier, stopping unrequested traffic from the internet reaching your devices.

How to tell what you have

Why it matters when things go wrong

Knowing which box does what tells you where a problem probably lies.

• Nothing in the house can get online: likely the modem, the line or the provider. Check the lights on the box; a red or flashing internet light often means the line is down.
• Devices on a cable work, Wi-Fi does not: likely the router's wireless side.
• One device cannot connect: usually that device, not either box.

1. If you have two boxes, switch off the router, then the modem or ONT.
2. Wait about 30 seconds.
3. Switch the modem or ONT on first and wait until its lights settle.
4. Then switch the router on and give it a couple of minutes.

Tip: Log in to your router's settings page (the address is usually on the sticker) and change the admin password from the default. Also check for software updates. An out-of-date router is one of the easiest ways into a home network.

Should you buy your own?

Most people are fine with the box their provider sends. Buying your own router can make sense if Wi-Fi does not reach every room, or if the provider's box is old. If you do, check it supports a modern Wi-Fi standard and still receives security updates from its maker. Keep the provider's box in case you need to test the line with them.

Want the technical version? See Network devices explained.