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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11The quickest way to check an Ethernet cable’s speed is to read the negotiated link rate between your computer and the router or switch. A 1.0 Gbps (1,000 Mbps) reading means that link has negotiated at gigabit speed; it does not prove that your internet connection delivers 1 Gbps. For actual wired-network throughput, run iPerf3 between two devices on the same LAN.
What the speed reading actually measures
Your operating system reports the rate negotiated by the Ethernet adapter and the port at the other end of the cable. Typical readings include 10 Mbps, 100 Mbps and 1.0 Gbps. This is a local link measurement, not an ISP speed-test result. Internet performance also depends on the router’s WAN connection, your ISP, remote servers and congestion.
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Use a link-rate check to find out what the endpoints agreed to. Use iPerf3 when you need to measure data moving across your local network and keep the ISP out of the test.
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Check the negotiated rate in Windows
- Connect the computer to a LAN port with an Ethernet cable. Turn off Wi-Fi if you want to isolate the wired path.
- Press Windows key + R, enter
ncpa.cpl, and press Enter. - Right-click Ethernet (or Local Area Connection) and choose Status.
- Read the Speed field. Record the exact value, such as 100 Mbps or 1.0 Gbps.
The value is the negotiated connection between that computer and the router or switch port. It is not the speed available from the internet.
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Check the negotiated rate in macOS
- Open Apple menu > System Settings > Network.
- Select the active Ethernet service and click Details.
- Open Hardware.
- Read Speed and Duplex.
A full-duplex gigabit result indicates that the Mac and the connected network port negotiated the expected wired mode.
Check the negotiated rate in Linux
Linux uses ethtool. First identify the wired interface; names vary by distribution and hardware. ip link lists interfaces, and a wired device commonly has a name such as eth0 or enp3s0. Then run:
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ethtool <interface>
Replace <interface> with the active wired name. In the output, check Speed, Duplex and whether Link detected is yes. A result such as Speed: 1000Mb/s describes the local Ethernet link.
Link-rate check versus an iPerf3 throughput test
| Method | What it measures | ISP involved? | Devices required | What it can isolate |
|---|---|---|---|---|
| Operating-system status | Negotiated rate between one adapter and the connected router or switch port | No | One computer and its Ethernet connection | Whether the link negotiated at 10, 100, 1,000 Mbps or another supported rate |
| iPerf3 | Measured end-to-end LAN throughput, with loss and related parameters | No | Two devices on the same LAN | Combined effects of cables, ports, adapters, hosts and software under the test conditions |
| Public internet speed test | Throughput over your LAN, router WAN path, ISP and remote test server | Yes | One device and an internet connection | Overall internet performance, not a cable by itself |
ESnet’s iPerf3 documentation describes the tool as performing active measurements to determine the maximum achievable bandwidth on IP networks. Because both endpoints are local, the test avoids turning an ISP speed result into a cable diagnosis.
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Measure local throughput with iPerf3
- Choose two capable devices on the same LAN, preferably both connected by Ethernet. Avoid a Wi-Fi hop unless you are deliberately testing it.
- On device A, start the server with
iperf3 -s. - Find device A’s LAN IP address. On device B, run
iperf3 -c <device-A-IP> -t 20, replacing the placeholder with that address. - Record the reported throughput and any loss or other parameters. Keep both devices awake and note CPU load and each adapter’s negotiated speed.
- Run the test in the reverse direction when upload and download behavior may differ.
Use matching iPerf3 versions on both devices; TP-Link’s cross-platform instructions warn that mismatched versions can prevent a test from working. A result is meaningful only when you record the setup and repeat it under the same conditions. There is no universal pass/fail throughput number for every cable, host, protocol and link rate.
What to do when the result is lower than expected
100 Mbps instead of 1 Gbps
Inspect every part of the path: the computer’s adapter, cable, wall jack, router or switch port, and any dock or adapter in between. Confirm that each endpoint and port supports gigabit Ethernet. Replace the cable temporarily with a known-good Cat6 Ethernet cable (or better) and repeat the link check. Cat6 guidance supports it as a sensible replacement for gigabit troubleshooting, but a cable swap alone does not prove the original cable was the only fault.
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1 Gbps link, but slow internet
The cable may be working normally. Run the two-device iPerf3 test first. If local throughput is healthy, investigate the WAN connection, router, ISP path or the internet test server rather than replacing the cable.
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- Verify both hosts’ adapter capabilities and negotiated rates.
- Check CPU utilization; a busy or low-power host can limit results.
- Temporarily account for security software or filtering that may inspect traffic.
- Check port negotiation and substitute a known-good cable and switch/router port.
- Change one variable at a time, then repeat the same command and duration.
A low iPerf3 number identifies a problem in the tested path, not a single failed component.
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Intermittent connection or suspected wiring fault
An Ethernet cable tester can check continuity and wiring when links drop or behave inconsistently. Choose a tester that matches the connector and wiring you need to verify. A tester complements, but does not replace, the negotiated-rate and throughput checks.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to interpret the complete result
- Expected link rate and strong iPerf3 throughput: the local wired path is behaving normally under the recorded conditions.
- Low negotiated rate: focus on endpoint capabilities, cable, jack and port before testing the internet.
- Expected negotiated rate but weak iPerf3: investigate host load, security software, duplex/port behavior and other path components.
- Healthy LAN but weak internet: troubleshoot the WAN or ISP path.
TP-Link’s documentation, last updated July 6, 2026, emphasizes that the displayed link speed is the negotiated Ethernet rate, not internet speed. The iPerf3 project documentation was current on October 17, 2026; use those readings as distinct measurements rather than substituting one for the other.
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