My girlfriend and I have a 1 Gbps internet plan. Her computer is connected directly to the router and reaches the expected speed. My PC is in the next room using a 50-foot Cat 6 cable from another router port, but Windows reports an Ethernet link speed of only 100/100 Mbps rather than gigabit. This is the negotiated link speed itself, not merely a disappointing speed-test result.
I have updated the Ethernet drivers, tried another cable, switched router ports, and checked the usual network settings. The cable works at gigabit speed when connected to her computer, and my motherboard is supposed to support gigabit Ethernet. I'm running Windows 11 and have already ordered a USB gigabit Ethernet adapter, although I'd prefer to understand what is wrong with the built-in port. What else should I check?
4 Answers
A gigabit connection needs all four twisted pairs. If one or two conductors are damaged, poorly terminated, or not making contact inside the PC’s Ethernet jack, the connection commonly falls back to 100 Mbps. A cable tester can confirm the wiring, but because the same cable reaches gigabit speeds on the other computer, inspect your PC’s Ethernet socket for bent contacts or damage. A USB 3.0 gigabit adapter should bypass that hardware completely.
Some motherboard Ethernet controllers have firmware or compatibility problems even when the board is advertised as gigabit-capable. Check the exact motherboard model for a LAN firmware or BIOS update, and verify that the adapter’s advanced settings allow 1 Gbps rather than limiting it to 10/100. If Windows won’t offer a 1.0 Gbps option or the USB adapter immediately links at gigabit, the built-in controller is almost certainly the issue.
Since the link itself is negotiating at 100/100 Mbps, the problem is probably between the PC’s network adapter and the cable—not your internet service. Set Speed & Duplex to Auto Negotiation, then completely remove the Ethernet adapter and driver, reboot, and install the latest driver from the motherboard manufacturer. If it still negotiates at 100 Mbps, the onboard NIC or its physical port is likely failing and replacing it with a gigabit adapter is the practical fix.
The cleanest troubleshooting step is to eliminate variables without changing settings: connect your PC to the exact cable and router port normally used by the other computer. Then connect the other computer using your original cable and port. If the other computer still links at 1 Gbps while yours stays at 100 Mbps, the PC’s Ethernet hardware or driver is the culprit. If both systems drop to 100 Mbps on one particular cable or port, that points to the cable termination or router port.
I already swapped router ports, but testing both computers with the same physical cable and connection is a better direct comparison.
That makes sense. The Windows status is definitely showing 100/100 rather than just getting 100 Mbps in a speed test, so I’ll focus on the adapter and port itself.