Why is my ethernet connection only running at 100Mbps?
A wired link that works but runs at a tenth of the speed is a classic single-pair fault. Why ethernet degrades rather than failing, and the four things that cause it in a home.
- Difficulty
- intermediate
- Time
- 25 min
- Read
- 3 min
Short answer
Gigabit ethernet uses all four pairs in the cable; 100 Mbps uses only two. Break one conductor in either of the pairs that gigabit needs and the link does not fail — it silently drops to 100 Mbps. So a connection stuck at 100 is almost always a cable, a wall socket or a plug, and almost never a router setting.
This fault is confusing precisely because everything works. There is no error, no dropout, no warning: files just copy slowly and the speed test never exceeds about 94 Mbps regardless of what you pay for. The cause is built into how ethernet negotiates, and once you know that gigabit needs four pairs and 100 Mbps needs two, the diagnosis becomes almost mechanical.
Step by step
- Confirm the link speed rather than the throughput.On Windows, open the adapter status and read the speed field. On a Mac, look at the network settings for the ethernet interface. On a router, the port status page usually shows the negotiated speed of each port. A reported 100 Mbps confirms the diagnosis; a gigabit link that transfers slowly is a different problem entirely.
- Swap the patch lead first.It is the easiest variable and the most common failure. Use a different known-good lead, ideally one you can see is not damaged, and recheck the link speed.
- Look into the plug.Hold the plug up to the light and look at the eight contact positions through the clear body. Cheap or old leads sometimes have only four conductors fitted, with four positions visibly empty. That cable physically cannot do gigabit.
- Try a different port at both ends.Router ports fail individually, and some routers have one port that is deliberately slower or reserved. Move both ends and see whether the fault follows the cable, the port, or the device.
- Suspect the wall socket if the run is fixed.A punched-down socket with one wire in the wrong position, a wire that has come loose, or two ends punched down to different wiring standards will all give this exact symptom. Re-terminate both ends to the same standard, or have it done.
- Check for physical damage along the run.A cable crushed by a door, pinched by furniture, stapled too tightly, or kinked round a sharp corner can break a single conductor while the outer sheath looks perfect. Follow the route and look at the pinch points.
- Check the device end.A very old computer, a cheap USB adapter, or an old network switch in the path may only support 100 Mbps in the first place. A 100 Mbps switch anywhere in the chain caps everything downstream of it.
- Rule out power saving.Occasionally a network adapter driver is set to drop the link speed to save power. In the adapter's advanced settings, set the speed and duplex to auto-negotiate rather than a fixed value, and disable green ethernet or energy-efficient ethernet if the fault persists.
If it doesn't work
Fast on one wall socket, slow on another
Cause: The slow socket is mis-terminated or damaged — Fix: Re-terminate that socket and its far end to the same wiring standard. Both ends must match; either standard works as long as it is used consistently.
Slow only with one particular cable
Cause: A broken conductor or a four-conductor cable — Fix: Retire the cable. Do not keep it in the drawer — it will be plugged in again by somebody in three years.
Everything on one switch is limited
Cause: The switch itself is a 100 Mbps model — Fix: Check the specification. Old switches look identical to gigabit ones. Replacing it with an unmanaged gigabit switch costs very little.
Drops to 100 only after the machine has been on a while
Cause: Power saving on the adapter, or heat — Fix: Disable energy-efficient ethernet in the adapter's advanced settings and retest over the same period.
Questions people ask
Why does it not just stop working?
Because the negotiation between the two ends finds the fastest mode both can manage with the pairs that are actually working. Two good pairs is a valid 100 Mbps link, so that is what it settles on.
Is 100 Mbps enough anyway?
For a single device browsing and streaming, usually yes. It becomes a problem when your broadband is faster than 100 Mbps, when several devices share that link, or when you copy large files across the house.
Could it be a router setting?
Very rarely. Some routers let you fix a port's speed manually and someone may have done so, but the default is auto-negotiate and the physical cable is far more likely.