What Is an ONT? Everything You Need To Know

An Optical Network Terminal (ONT) is a device that converts fiber optic signals from your internet service provider (ISP) into digital ones. This process makes it so devices like your router can understand fiber optic signals.
What Is an ONT

As a network enthusiast, I want to understand how networking devices work fully. This knowledge helps me optimize my network connections.

I want to help you optimize yours, too. So, I put together all of the information I could regarding ONTs.

To fully understand what ONT is, I’ll go over areas like:

  • What it is
  • Differences between ONT and other devices
  • How it works
  • What you’d use it for
  • Other important information

Keep reading to learn more.

What Is an Optical Network Terminal?

An ONT, or fiber network terminal, is a type of customer-premises equipment (CPE). In most scenarios, it’s a small white plastic box that you’ll use to connect your outside network box.

It will connect your ISP’s optical network with your local area network (LAN). It’ll convert fiber optic broadband signals into phone, Wi-Fi, and Ethernet connections.

Without ONT, you can’t use fiber optic internet speeds. So when subscribing to a fiber optic, ISPs will provide you with one of these devices. They’ll also send a technician to your home to install them.

Some ISPs will combine their ONT with Wi-Fi routers. The resulting device gives you something similar to a gateway device. Devices that serve as entry- and exit points for your network.

You can still use your own router in most cases, but you’ll need to put your ONT into bridge mode. Otherwise, with two routers on the same network, you run into the risk of Double network address translation (NAT).

This leads to IP address conflicts and can sometimes lead to performance issues.

In simpler terms, you’ll turn off the ‘routing’ part of your router with bridge mode.

What Does an ONT Look Like?

As I mentioned earlier, most of them are little white boxes. In this case, I could only find a black one. 

An optical network terminal without any cables.

They look like routers. If you look on the right side of the left image, you’ll see ‘PON.’ That stands for passive optical network. You’ll use this port for your fiber connection.

Many ONTs have more ports on the back.

Take a look through this user guide [1]. It’ll show you an example of a fiber network terminal that has a couple of phone ports and several LAN ports.

Then you’d have a LAN port that you’d use to connect directly to your router.

Like modems and routers, ONTs also have lights that indicate your network’s status. The power light states on your device will vary by model. In general, you’ll see:

  • Loss of signal (LOS)
  • Passive optical network (PON)
  • Wide area network (WAN)
  • AUTH
  • LINK

You don’t want these lights flashing or off. That shows there’s a problem with your network.

ONT vs. Modem

A modem’s short for Modulator Demodulator. It serves as a term used for device classes like telephone, DSL, and cable modems. It also will convert your internet’s digital signals to analog radio frequencies (RF) (and vice versa).

0’s and 1’s represent this signal translation.

Optical Network Terminals are modems, but only for fiber optic internet.

Both devices directly communicate with your internet service provider. Instead of using a traditional coaxial cable, an ONT will use a fiber optic cable. This cable allows your device to communicate with your ISP using infrared light pulses.

When using an ONT, you won’t need a modem. Instead, you can plug your devices directly into your Optical Network Terminal device.

What’s the Difference Between an ONT and an ONU?

Unlike the ONT, which sits inside your home, you would find an optical network unit (ONU) outside. An ONU will use fiber cables to convert optical signals to electrical. 

It’ll then organize and enhance various data types and send them upstream to the optical line terminal (OLT). An OLT serves as an ISP’s endpoint in a PON. It’s your ISP’s equipment.

In a way, ONTs and ONUs are the same. They both receive signals from your OLT.

How Does an ONT Work?

Your ISP’s optical line terminal will send data through the passive optical splitter. From there, fiber optic cables will transmit data downstream and upstream to your ONT or ONU.

If you connect devices to your ONT with Ethernet, then you’ll receive internet from that. Or, you connect your ONT to your router, which will then transmit wireless signals throughout your home.

An infographic that shows how an optical line terminal transfers data to an optical network terminal.

What Is an ONT Used For?

You’d use ONT in fiber to the business (FTTB) or fiber to the home (FTTH). They’ll perform a few different actions:

Telephone power: some ONT plans can include a battery backup. This service makes it so you can receive or make calls during a power outage.

Terminates fiber optic lines: delivers fiber optic signals from the ISP’s central office to your business or home.

De-multiplexes fiber optic signals: fiber optic signals will come through lines as a multiplexed signal. This means a single signal. An ONT will divide that signal into several parts; Internet, phone, and television.

How Do You Connect an Optical Network Terminal?

A technician will install the ONT into your home. So they’ll do most of the work for you. When connecting it to other devices, you still don’t have to worry about much.

It works like any modem. Take one end of an Ethernet cable and plug it into a LAN port on the back of your ONT.

Take the other end of the cable and plug it into your router or device.

If you’re in a situation where you can’t move your Optical Network Terminal but want an Ethernet connection, there’s hope.

Powerline adapters.

These devices come as a pair. You’d plug one into an outlet near your ONT and another into an outlet near a device you want an Ethernet connection. Next, use two Ethernet cables. 

Connect your ONT and first Powerline adapter with one. Then your device and the second adapter with the other cable.

These devices will use your home’s electrical wiring to send internet through your home—as long as the wires use the same circuit.

When considering Powerline adapters, you’ll need to understand they have weaknesses like:

  • They support up to 200 Mbps [2]
  • They’re inefficient in homes with poor electrical wiring
  • Powerline adapters with other brands may not work with each other

If you’re in a home that you don’t want to run wires through and has thick walls, these devices will work well.

Where Should I Put an ONT?

Before your ISP’s technician arrives to install your ONT, you’ll need to know where you’ll want it. Otherwise, if you’re going to move it later, you’ll need to call another technician. Thus, moving your ONT will cost you more money.

Here are some factors to keep in mind when determining where you’ll place your Optical Network Terminal:

  • Keep it away from a busy area: prevents anyone from bumping your device
  • Near an outlet: unless you want to get an extended power cable
  • Close to where you’ll use internet: for an optimal connection

Keep in mind that when technicians run your fiber cable to your External Termination Point (ETP), they’ll do so under the floor or your wall or roof cavities.

When considering where to place an indoor ONT, you’ll also want to know an optimal location for your router. Placing your router in the wrong spot could significantly lower internet speeds.

For example, I place my router in the center of my home and on a shelf.

That’s because routers radiate Wi-Fi signals around them like a sphere. This position lessens the likelihood of objects blocking my signal and prevents signals from covering areas I never use.

What Should I Do if My ONT Stops Working?

Check your ONT’s lights to see what isn’t working.

Your device stores short-term memory (cache) of network data. In rare scenarios, it may accidentally store errors, which can cause internet stability and speed issues.

You can reset your cache and eliminate these errors by manually rebooting your fiber network terminal. To reboot your ONT, you’ll need to unplug your device.

Some fiber network terminals will have a ‘Reset’ button. Don’t press it. Doing so will wipe your device’s settings.

Only press this button as a last resort.

There isn’t a universal rule on how long to leave it unplugged. But on AT&T’s forums, Community Support says to leave it disconnected for at least a few minutes [3].

If you have a battery backup unit (BBU), these stop working over time. Loose wiring, overheating, and an idle unit can all cause problems.

These problems will also affect your ONT.

First, you’ll want to unplug your battery backup and wait 30 seconds. Doing so will give your device a chance to cool.

Meanwhile, check the fan’s air filters. If they’re clogged, unclog them. Cleaning them will prevent future overheating.

Plug your BBU into the outlet and check for a green light. If you don’t see it, it’s not receiving power. You’ll need to test your outlet with a lamp to see whether it’s delivering power.

If so, that’s a sign you’ll need a new battery backup unit.

Do I Need an Optical Network Terminal?

You’ll need an Optical Network Terminal if you’re subscribing to a fiber to the premises (FTTP) plan. And if you do get an ONT, you can’t install it yourself.

Fiber optic termination, which is the process that makes it so you can plug fiber cables into your wall, requires special tools. You’d also need equipment to test whether you’re receiving fiber optic signals.


Optical Network Terminals and ONUs play a critical role in providing fiber optic internet services.

Your ISP will deliver fiber optic signals from their ONT and through a passive optical splitter. And eventually, your ONT will intercept the signal, de-multiplex it, and deliver your ISP’s services.

In a way, they’re a modernized modem.

If you upgrade to fiber optic services, you’ll need a router to deliver your higher speeds. We’ve gathered and reviewed many fiber optic routers. However, only seven of them stood out. Check out what ones we recommend.

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