The router and the modem are two of the easiest devices to get confused. They sit next to each other, they look pretty identical, and they both help you connect to the internet.

Despite having so much in common, they are actually opposites.

Your router faces inwards—it connects multiple computers in one network together. It creates a local area network (LAN), which is what you would call your internal network of computers in your office or home.

Your modem faces outwards—it’s what connects your LAN to the outside world, the Internet. Through your modem, your LAN connects with the wide area network (WAN), which extends over a large geographical distance, such as Manhattan or any metropolitan area.

Together, you have the basic setup to connect your small office or family home to the Internet. Here’s how.

What Does a Router Do?

A router has both a LAN port and a WAN port. It’s the bridge between your computer and your modem. If you’re not sure which box is your modem and which one is your router, routers usually have antennae for Wi-Fi.

The router allows multiple computers to connect to the same modem, which provides access to the Internet beyond. You don’t actually need a router to connect to the Internet, but you do need one if there’s more than one device on your network, or if you’re using Wi-Fi.

Conversely, if you have only a router and no modem, the devices on your LAN can still communicate. For example, I can send a file to my co-worker’s computer through the LAN. Without a modem, however, neither of us can get online.

How it Works

A router forwards data packets between computer networks. It performs “traffic directing” on your network, sort of like a traffic director on an airplane runway directs planes to the appropriate terminals. But instead of terminals, it’s using IP addresses.

Your router itself has multiple IP addresses. It has a public IP address, a private “management” IP address, and then additional private IP addresses for each device in addition to the router’s internal IP, which is your LAN default gateway. If there’s only one computer in your office, your router has a total of two IP addresses: one public and one private.

These enable the router to forward packets of data destined for the Internet to the modem, and to the data’s destination IP address outside your office. The router also keeps internal traffic—computer to computer, computer to printer, etc.—from leaving the LAN.

When relaying signals to the modem and the Internet beyond, your router makes your individual computer’s IP address invisible. It only presents its public IP address. So if you’re connecting to the web from your office, the site you’re visiting will see that someone from your company is accessing, but they won’t be able to see that it’s you specifically.

However, when sending information back to computers inside the LAN, your router knows exactly what device to send specific data to. It uses each device’s unique IP address to keep a record of which computer made a request to the Internet and when. The router assigns these IP addresses to each computer that requests to connect to it based on the device’s physical MAC address.

What Does a Modem Do?

Your Internet Service Provider (ISP) can’t communicate directly to your computer. Those wires coming out of the wall socket would have to connect directly into some sort of port directly on your computer, and your computer would need to have the corresponding hardware. Your modem solves that problem—it’s the go-between.

Modem: Mo (modulator) + dem (de-modulator). It’s a machine that converts digital signals to analog and back again. Data that travels over phone or cable lines is analog, but your computer only understands digital. The modem makes that translation.

How it works

In the 1990s, dial-up modems had to actually “call” the Internet before connecting through the phone line. Today, DSL and cable modems are able to use dedicated data lines. They convert signals to travel across fiber optic wire, which is much faster. We’re going to see a lot more fiber optic wire getting installed in coming years.

Remember that your modem is mostly “outward facing.” It communicates with your ISP, and out across the vast cyber-expanse of the Internet beyond your office LAN.

If you ever notice your modem blinking wildly, it’s not necessarily because there’s a lot of traffic on your internal office network. It’s because of all the activity happening out there on the web. Your ISP and other agents are constantly scanning your ports to verify your IP address.

So be grateful for the blinking. It means your modem is receiving and responding to signals from your ISP.

Putting It All Together

When you connect to a router, you’re getting access to a local network—but that doesn’t provide you access to the Internet. Your router needs to be connected to a modem in order to tap into the Internet. The modem converts digital signals from your computer to analog signals that can be sent across long distances through wire.

But if you just want to print a document via a printer that’s connected to your LAN—without being hooked up to the Internet—you can. As long as both devices are recognized by your router.

When your computer sends a packet of data to the Internet, the packet goes to your router first. Like the reliable traffic controller that it is, your router sends the packet to your modem, and your modem connects it to your ISP. So the path a packet of datatakes looks like this:

  1. Your computer
  2. Router
  3. Modem
  4. ISP
  5. Internet

Combined Routers and Modems

Routers and modems are usually separate entities, but sometimes your ISP will offer a combined device. This simplifies the setup process, connects you to the Internet, and provides a local wi-fi network.

Some ISPs like offering two-in-one devices, but there’s no reason you have to sign up. The combined device works for basic use, but if you’re doing heavier, more advanced networking stuff, or if security is important for your business, you should get separate units. Separated routers typically offer better security functions.

If you end up renting hardware from your ISP, you might see an additional “rental fee” charge on your monthly bill statement. It might actually be more cost-effective for you to buy the equipment outright—so do the research, or call your ISP for more info.