Ethernet Stuck at 100 Mbps? How to Get Your Gigabit Speed Back

You pay for a gigabit plan. Your PC is plugged straight into the router with a cable. And downloads crawl along at just under 100 Mbps, while your phone pulls three or four times that on wifi from the couch.

This one drives people up the wall because it feels backwards. The cable is supposed to be the fast option. Here's the thing though. When a wired link sits at exactly 100 Mbps, the cause is almost always physical, cheap to fix, and findable in an afternoon.

Check the link speed before you blame your ISP

Don't start by rebooting things at random. Start with the number your computer reports for the connection itself. That number tells you where the problem lives.

On Windows 11, open Settings > Network & internet > Ethernet and find Link speed (Receive/Transmit). If it reads 100/100 (Mbps), you've found your problem. Your computer and router agreed to talk at 100 Mbps, so nothing else in the chain can go faster.

On a Mac, the same figure sits in System Settings > Network > Ethernet, under the hardware details. Game consoles bury a similar number in their network status screens.

A healthy gigabit link reads 1.0 Gbps. If yours already shows that, the cable isn't your problem. Run our speed test a few times, and if results still disappoint, work through our slow internet fixes instead.

Why the number is exactly 100

Gigabit ethernet needs all four wire pairs inside the cable, which means all eight pins in the plug. The older 100 Mbps standard, 100BASE-TX, only uses two pairs. That gap explains nearly every case of this problem.

When your PC and router link up, they negotiate the fastest speed both ends can manage. If even one of the eight conductors is broken, bent, or not touching, gigabit is off the table. Both ends quietly drop to 100 Mbps, because that still works on the wires that remain.

There's no error message and no warning light. The link comes up, the internet works, and everything runs at a tenth of the speed you're paying for. People live with this for months without noticing.

It also explains the classic mystery of ethernet running slower than wifi. A midrange phone on 5 GHz wifi can negotiate several hundred Mbps. Your damaged cable is pinned at 100. Same router, same internet plan, very different links.

Swap the cable first

The most common cause is the cable itself, and it fails in two ways. The first is age. Plain Cat5, not Cat5e, was built for the 100 Mbps era and often can't hold a stable gigabit link. Read the printing on the cable jacket, and if it says Cat5 with no e after it, retire the cable.

The second is damage. A chair wheel, a pinched doorway, a staple, or a bored pet can sever one conductor and leave the other seven intact. The cable looks perfect from the outside. It still carries 100 Mbps happily, so nothing ever seems broken.

So do the cheap fix first. Swap in a known-good Cat5e or Cat6 cable and check the link speed again. Cat5e handles gigabit fine, so there's no need to buy anything exotic. And if you're wondering whether wired is even worth the fuss, our ethernet vs wifi comparison settles that.

Inspect the plugs and ports

If a fresh cable changes nothing, look at the metal. Shine a light into the ethernet port on the computer and on the router. The eight gold pins should sit in a neat, even row. One pin bent flat or pushed down will force a 100 Mbps link.

Dirt and oxidation do the same job more slowly, since contacts tarnish over time. Plug and unplug the connector several times to scrape the contacts clean. A puff of air clears the dust out of the port.

Check the plastic clip on the cable too. A snapped clip lets the plug sag in the socket, and sagging pins lose contact. Then try a different LAN port on the router. Ports die one at a time, and the one you've always used may be the casualty.

Fix the adapter settings on your computer

Settings cause fewer of these cases than hardware does, but they're worth five minutes. Open Device Manager, expand Network adapters, right-click your ethernet adapter, and pick Properties. Then open the Advanced tab.

Find Speed & Duplex and make sure it says Auto Negotiation. Intel's own support guidance is blunt about this. Auto-negotiation gives you 1.0 Gbps whenever the cable and ports are healthy, so leave it alone under normal conditions.

You can force 1.0 Gbps Full Duplex as an experiment. If the link dies completely when you force it, the wiring truly can't carry gigabit. That confirms a physical fault. Forcing the speed isn't a fix though, so set it back to Auto Negotiation afterward.

While you're in the Advanced tab, hunt down the power-saving options. Energy-Efficient Ethernet, Green Ethernet, Gigabit Lite, and Power Saving Mode all trade link speed for lower power draw. Any one of them can downshift the link. On a desktop PC, turn them all off, since the savings amount to pennies.

Old drivers cause the strangest cases. Get the newest ethernet driver from your PC or motherboard maker's website instead of trusting Windows Update alone. Most home machines use Realtek or Intel adapters, and both post fresh drivers on their own sites.

The slow port might be in another box entirely

Your PC reaches the router through whatever sits in between. Every one of those boxes has its own ports. A single Fast Ethernet port anywhere in the path caps everything behind it at 100 Mbps. It doesn't matter how good the rest of the chain is.

The usual suspects are boxes you stopped thinking about years ago. An old ISP router. A small desktop switch from a previous decade. Powerline adapters deserve special suspicion, since plenty of budget kits use 100 Mbps ports whatever the box advertises.

Cheap USB ethernet dongles are another trap, because many of them are Fast Ethernet only. In-wall wiring counts too. Someone terminated those wall jacks by hand, and one loose conductor at a jack creates the same 100 Mbps ceiling.

Corner the culprit one swap at a time

When nothing obvious stands out, isolate the problem the boring way. It's slower than guessing, but it actually ends the mystery.

1. Plug the PC straight into a router LAN port using the suspect cable. No switch, powerline adapter, or wall jack in between.
2. Check the link speed. If it now reads 1.0 Gbps, whatever you just bypassed is the culprit.
3. Still 100 Mbps? Swap in your known-good Cat5e or Cat6 cable and check again.
4. Still stuck? Move to a different LAN port on the router.
5. Still stuck? Try a different device, like a borrowed laptop, on the same cable and port.

The component whose replacement flips the link to 1.0 Gbps is your answer. Replace that one thing and stop. There's no reason to rebuild the whole network over one tired cable.

Once you're back at full speed, glance at the link speed after any change to your setup. Moved the PC or added a switch? Check it. The check takes ten seconds, and catching a downshift on day one beats months of losing to wifi.