What is the ONT on my wall, and what do its lights mean?
The small box a full fibre installation leaves on your wall, why it is separate from the router, and which of its lights actually tells you whether the fibre is working.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 3 min
- Safety
- caution
Short answer
The ONT — optical network terminal — converts the light arriving down the fibre into ordinary ethernet, and hands that to your router by a normal network cable. It is the provider's equipment. The light labelled optical, PON or fibre is the one that matters: steady means the fibre side is fine and any fault is beyond it, while off or flashing red means the fibre or the provider network, and restarting your router will not help.
Full fibre installations leave people with two boxes where they used to have one, and no explanation of which does what. The consequence is that when something goes wrong, people restart the router — the one box that is almost certainly not the problem — and never look at the one that is actually reporting the fault. Ten minutes understanding this layout saves an hour every time.
Safety
Step by step
- Identify the boxes.A small box with a fibre lead into it and a single ethernet lead out is the ONT. The box with the wifi aerials and several ethernet ports is the router. Some providers combine both into one unit, in which case there is only one box and no separate ONT.
- Trace the path.Fibre from outside enters at a small entry point, usually with spare fibre coiled inside it. A fibre lead goes from there to the ONT. An ordinary ethernet cable goes from the ONT to the router's WAN or internet port. Everything else in the house hangs off the router.
- Learn the lights.Typically power, optical or PON, and LAN, sometimes with an alarm indicator. Power is self-explanatory. Optical or PON reports the fibre link to the provider. LAN reports the ethernet connection to your router. Exact labels and colours vary by model, so check your provider's page for yours.
- Read the optical light first when something goes wrong.Steady green or blue normally means the fibre is fine. Off, red, or slowly flashing normally means no signal from the provider. That single light divides every fault into two categories and tells you which half to work in.
- Check the LAN light next.If optical is healthy but LAN is off, the cable between ONT and router is unplugged, damaged, or in the wrong port on the router. That is a fault you can fix.
- Leave the ONT powered.It has its own power supply and it is not meant to be switched off. Turning it off does not help ordinary faults, and some providers monitor its status remotely — an ONT that keeps disappearing looks like a fault at their end and can trigger unnecessary visits.
- Protect the fibre.Do not move furniture against it, do not staple it, and do not coil it tightly to tidy it. A single sharp bend can break the glass inside a lead that looks undamaged from outside.
If it doesn't work
Optical light red or off, no internet anywhere
Cause: The fibre link or the provider network — Fix: Check the provider's status page from mobile data, then report it. Do not dismantle anything at home — nothing you can do at your end affects this light.
All ONT lights normal, no internet
Cause: The router, or the connection between it and the ONT — Fix: Check the ethernet cable is in the router's WAN or internet port and not a LAN port. Restart the router, not the ONT.
Alarm light lit
Cause: The ONT has lost its configuration or reports a fault — Fix: Report it with the exact light pattern. Note what each light is doing before you ring — it is the first thing you will be asked.
Everything works but the ONT is warm and noisy in a cupboard
Cause: Poor ventilation — Fix: Give it air. It is a small always-on device and heat shortens its life, though a little warmth is normal.
Questions people ask
Can I use my own router with an ONT?
Usually yes, and it is much simpler than on older connection types because the ONT does the modem job. Your router plugs into the ONT by ethernet. Some providers require specific settings on the router, so check their support pages first.
Do I still need a microfilter?
No. Microfilters exist to separate broadband from voice on a shared copper pair. Full fibre has no such pair and no filter is used anywhere.
What happens to my phone line?
On full fibre, landline calls are carried over the broadband, usually by plugging a phone into a socket on the router. That means the phone stops working in a power cut, which matters if anyone in the house relies on it — ask the provider about their arrangements for that.