These are the author's reported observations, not independent measurements. They still make a useful, repeatable check for any Windows PC.
Link speed versus internet speed
Link speed is the negotiated rate of the wired connection between your PC's network adapter and the device at the other end of the cable, usually a router or switch. It isn't your ISP plan speed. Typical values are 100Mbps, 1Gbps and 2.5Gbps, depending on the hardware.
The two devices normally settle on the fastest mode both support, a process called auto-negotiation. Because it is automatic, a bad downgrade can go unnoticed. Nothing pops up to say your PC gave up on gigabit.
The practical consequence is simple. A 100Mbps link caps wired throughput below 100Mbps, so a 150Mbps plan can never show its full speed. In the author's setup, that matched what he saw.
The reverse doesn't hold. A healthy 1Gbps link has plenty of local capacity for a 150Mbps plan. It doesn't prove the ISP is delivering that speed.
How to check the negotiated speed
The author's route uses the classic Windows screens:
- Open Control Panel, then Network and Internet, then Network and Sharing Center.
- Click Change adapter settings.
- Right-click your Ethernet adapter and choose Status.
- Read the value next to Speed.
Microsoft's current support guidance for Windows 10 and 11 starts at Settings, then Network & internet. That page shows connection status and properties for Ethernet. Exact labels and screens vary by Windows version, and the classic Status dialog is the one that showed the author's link speed. If your adapter shows 1.0Gbps and your speed test is still low, the cable isn't the explanation.
Check Speed & Duplex, but leave it on auto
The author also checked the adapter's advanced setting:
- Open Device Manager and expand Network adapters.
- Right-click your Ethernet adapter and choose Properties.
- Open the Advanced tab.
- Select Speed & Duplex and look at the Value menu.
He confirmed it was set to auto-negotiation, and that is the right default. Gigabit copper Ethernet relies on auto-negotiation, and forcing a mode can break the link or cause fallback. Troubleshooting guides found in research say the same. Sherafy's guide advises returning the setting to Auto Negotiation unless you have a specific reason not to. Property names and available values depend on your adapter and driver, so yours may look different.
Why a damaged cable can still work at 100Mbps
Ethernet cables normally contain eight copper conductors twisted into four pairs. 100Mbps Ethernet can run on two pairs, but Gigabit Ethernet needs all four. If one pair is damaged, incorrectly terminated or not connected, the devices may still establish a link but negotiate it at 100 Mbps instead of 1 Gbps. That is why such a cable can look healthy. It carries traffic and the connection stays up, just slowly.
The author's cable was old and had been rolled over by office chairs and caught in a door. Damaged internal wiring is a plausible explanation. The swap alone doesn't prove it, though. It shows that something in the old cable, or its connectors, was the problem. It doesn't show which conductor or contact failed.
A WindowsForum report from about three weeks ago described a similar case. A long flat cable run kept dropping speeds, and replacing the cable fixed it. That write-up also cautioned that a link stuck at exactly 100 Mbps is a strong cable clue, while a 2.5GbE port settling at 1 Gbps is a weaker one. A fault that comes and goes can make the pattern messier.
A step-by-step isolation routine
Change one thing at a time, and write down the Windows link speed after each change.
- Note the current link speed in the adapter Status screen.
- Confirm both ends are gigabit-capable. A 100 Mbps LAN port on a router, or a similarly limited USB Ethernet adapter, will limit the connection no matter how costly your Ethernet cable is. HP's community guidance notes that some desktop models ship with 10/100-only ports.
- Reseat both ends of the cable.
- Swap in a known-good cable that is Cat5e or better. This is what fixed the author's problem.
- Try a different router or switch port.
- Remove intermediate hardware if you can. That means wall jacks, patch panels, couplers, hubs and switches. A badly punched-down wall jack can break one pair and still leave a 100Mbps link running.
- Try another gigabit device on the same cable. If both negotiate at 100Mbps, suspicion shifts to the cable, termination, switch or upstream port.
- Run a speed test only after the link shows the expected rate.
If the link is still at 100Mbps after a verified cable, a confirmed gigabit port, auto-negotiation and updated drivers, the likely causes are faulty hardware or building wiring. For in-wall runs, a cable tester is worth borrowing. Never trust the printed label alone.
Cat5e, Cat6 and the buying advice
Two points in the original piece need tightening.
First, Cat5e is rated for Gigabit. The author reached 1.0Gbps with a new Cat5e cable, so he didn't need Cat6 to fix this. The cause was a faulty cable or connection, not the cable category.
Second, he argues that Cat6's 250MHz bandwidth gives headroom for 10Gbps NAS transfers. That overstates it. Cat6 supports 10Gbps only over short runs. One industrial-networking reference lists 55 meters for 10GBase-T on Cat6, and Cat6A is the category aimed at full 100-meter runs. The cable also isn't the only limit. Your NAS, adapter and switch all have to support the speed. Cat6 is a reasonable buy for future-proofing, but it won't deliver 10Gbps without matching hardware.
The author is also loose about earlier categories. Cat5 is the category associated with 100Mbps Ethernet, but the cable label doesn't tell you what a specific installed run will do.
The takeaway
The lesson is cheap and unglamorous. When speed tests disappoint, check the local link before you blame the ISP or reset the router. One Windows dialog will tell you whether your PC is running at 100Mbps, 1Gbps or something faster. If it says 100Mbps on gigabit-capable gear, start with the cable.
A replacement cable is often only a few dollars. An hour on the phone and another hour in router settings cost more.