Modem or router: what is the difference and what does each do?

In short: the modem brings the internet into the home, the router distributes it among devices. Both are needed to connect several devices, but the internet box already combines them in a single unit. A separate router only becomes useful to improve Wi-Fi, add Ethernet ports or get more advanced settings.

Modem or router: both words often refer to the same box sitting in the living room, which keeps the confusion going. They are nevertheless two distinct and complementary functions. One connects the home to the internet provider’s network, the other creates the home network and shares the connection between the computer, smartphones, TV and connected devices. Understanding what each one does helps solve a connection problem, choose equipment or decide whether it is worth adding a router to the box.

Modem and router: the differences in one table

What is a modem?

The link with the internet provider

The modem is the device that establishes the physical connection between the home and the internet provider’s network. Without it, no device in the house can reach the internet. It receives the signal arriving over the line, converts it into data that devices understand, and does the reverse for everything leaving the home.

Modulator and demodulator

The word “modem” is a contraction of modulator and demodulator. The first modems translated the analogue signal of the telephone line into digital data, and vice versa. Today’s connections no longer work in the same way, but the name has stuck to describe the device that links to the operator’s network.

A modem for each type of line

The modem depends on the technology that brings the internet to the home.

• ADSL and VDSL: the connection runs over the copper telephone line.
• Cable: the connection arrives through a coaxial cable, the one used by former TV networks.
• Fibre optic: an optical network terminal, or ONT, translates the light signal of the fibre into a signal the home network can use.

What is a router?

A router is a device that links two or more networks and directs data from one to the other. At home, it connects the home’s local network to the internet, which it reaches through the modem. To route each data packet, it consults a routing table, a list of paths to the various destinations, then sends the packet to the right address.

What a router does at home

The router provides several everyday services without the user having to deal with them.

• It creates the home’s local network, over Wi-Fi, Ethernet cable or both.
• It shares the connection: thanks to it, every device uses the same internet connection at the same time.
• It assigns an IP address to each device on the network, through the DHCP protocol, while the home’s connection has its own address on the internet.
• It lets devices talk to each other: sending a document to the printer, casting a video to the TV, viewing a camera from a smartphone.
• It filters traffic with a firewall that protects devices from unwanted access from the internet.

Router, switch and access point

These three devices are often confused. A switch passes data between devices on the same network and mainly serves to add Ethernet ports. A router, on the other hand, moves data between different networks. A Wi-Fi access point broadcasts the wireless network, and it is built into most home routers.

Modem or router: are both needed?

To connect several devices to the internet, yes. A modem alone can connect a single device, for example a computer plugged in directly. A router alone creates a local network where devices exchange data with each other, but with no internet access at all. It is the combination of the two that allows the whole house to be connected at the same time.

In practice, the question rarely comes up in these terms. Almost every internet plan supplies a device that does both: the box.

The internet box, a modem and a router combined

An internet box is a modem router: it combines in a single unit the modem that receives the connection, the router that creates the local network and the Wi-Fi access point. It often adds a few Ethernet ports and, depending on the plan, phone and TV services. To the question “is my box a router?”, the answer is therefore yes.

This combination makes installation simpler: a single device to plug in, configure and update, and support from an internet provider that knows the hardware. Under the hood, however, the functions remain separate. That is what makes it possible to complement the box with a separate router when its capabilities are no longer enough.

When to add a router to the box?

For most households, the box is enough. A separate router, connected behind the box with an Ethernet cable, has its place in a few specific situations.

• The box’s Wi-Fi covers the home poorly, even when placed in the centre. A newer router or a mesh Wi-Fi system then takes over wireless broadcasting.
• There are not enough Ethernet ports to connect a TV, a games console, a desktop computer or a file server by cable.
• Needs go beyond the box’s settings: finer parental controls, a virtual private network (VPN), priority given to certain devices, a separate network for connected devices.
• The box’s Wi-Fi standard is dated: Wi-Fi evolves faster than modem technology, and a recent router makes it possible to enjoy a faster standard without changing plans.

Before investing, it is worth measuring the speed received in each room. The guide how many Mbps for a good Wi-Fi connection explains how to do it and what speed to aim for. If the problem only affects one or two rooms, extension equipment often costs far less than a router: the comparison powerline or Wi-Fi extender helps make the choice.

Securing a modem router

The box and the router are the front door of the home network. The French public service that assists victims of cybercrime recommends a few simple settings (Cybermalveillance.gouv.fr, how to secure a Wi-Fi network).

• Change the default Wi-Fi password to a long, complex and unique one.
• Change the password of the administration interface of the box or router, different from the Wi-Fi password.
• Enable WPA2 or WPA3 encryption in the wireless settings.
• Disable WPS and remote administration when they are not used.
• Install updates for routers, extenders and powerline adapters, which do not always update automatically.
• Create a separate network for guests and connected devices, such as cameras or smart plugs.

Where to place the box and the router

The router is usually installed next to the modem, or the box, so they can be linked with a short Ethernet cable. Since Wi-Fi handles most uses, the location matters as much as the device. Thick walls weaken the signal or even block it. Good habits: place the unit near the centre of the home, up high on a piece of furniture or a shelf, in the open rather than shut away in a cupboard. A Wi-Fi outdoor security camera, for example, depends directly on the quality of the signal where it is mounted.

The other guides in the Wi-Fi and networking section detail the solutions for extending wireless coverage throughout the home.