My girlfriend and I have a 1 Gbps internet plan. The router is next to her office, where her PC gets the full speed over Ethernet. A 50-foot Cat 6 cable runs from another router port to my PC in the next room, but Windows 11 reports an Ethernet link speed of only 100/100 Mbps. This is the negotiated link speed itself, not merely a speed-test result.
I have updated the motherboard's Ethernet driver, tried different router ports, and tested the same cable with my girlfriend's PC, which immediately gets gigabit speeds. My motherboard is supposed to support 1 Gbps Ethernet, and the adapter settings are not solving the problem. I have ordered a USB 3.0 gigabit Ethernet adapter, but I would prefer to understand what is wrong with the built-in port. What troubleshooting step or hardware issue might I have missed?
4 Answers
Since the same cable works at gigabit speed on another computer and you already swapped router ports, the likely problem is your PC’s Ethernet adapter or its physical jack. Check that Speed & Duplex is set to Auto Negotiation, then completely remove the network adapter and driver, reboot, and install the newest driver from the motherboard manufacturer. If the link still negotiates at exactly 100/100, the built-in NIC may be failing or have a firmware/controller issue. A USB 3.0 gigabit Ethernet adapter is a reasonable workaround and should confirm that diagnosis.
The cleanest test 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 to your usual cable and port. If the other PC still gets 1 Gbps but yours remains at 100 Mbps, the fault is isolated to your PC’s Ethernet port, NIC, or driver. A cable tester can also check for damaged conductors or a bad termination, although the successful gigabit test makes the cable much less likely to be the cause.
This kind of swap test is more useful than repeatedly changing router settings because it tests the cable, port, and computer independently. A gigabit connection needs all four twisted pairs, while a damaged pair can cause negotiation to fall back to 100 Mbps.
There are known cases where a motherboard’s integrated Ethernet controller develops a hardware or firmware problem and falls back to 100 Mbps even with a good cable. If a fresh driver installation and the swap test confirm that the onboard port is the only failing component, the USB gigabit adapter is likely the simplest solution. Use a USB 3.x port and verify that the adapter’s status reports a 1.0 Gbps link.
Check the motherboard model and confirm that its Ethernet controller is actually gigabit-capable; some systems, especially laptops or unusual motherboard revisions, only include 10/100 hardware. Also check the adapter’s Advanced properties. If the Speed & Duplex menu does not offer 1.0 Gbps Full Duplex, that points to a hardware, driver, or firmware problem. Forcing 1 Gbps usually is not a real fix—Auto Negotiation should work when the hardware and wiring are healthy.
The adapter status is already showing 100/100 Mbps, so this is definitely the negotiated link speed rather than a limitation from the internet connection. I’m on Windows 11 and have already tried switching router ports.