Wired troubleshooting
Ethernet stuck at 100 Mbps: the usual causes
Short answer: a wired connection that tests near 94 Mbps is almost always negotiating 100 Mbps, and the most common reason is a cable or connector that is not carrying all four wire pairs. Fast Ethernet (100BASE-TX) uses two pairs; gigabit (1000BASE-T) uses all four, so one broken pair can leave the link working but slow.
Why two pairs can work and four cannot
Wikipedia's article on Fast Ethernet notes that 100BASE-TX uses pins 1, 2, 3 and 6, which is pairs 2 and 3 (orange and green) in the T568A/T568B wiring standards (Fast Ethernet). Its article on Gigabit Ethernet says all four pairs are required for 1000BASE-T data communications, and that because autonegotiation takes place on only two pairs, two gigabit interfaces joined by a cable with only two working pairs will still complete negotiation and fall back to the best common mode (Gigabit Ethernet). In practice, a cable with a damaged or badly crimped pair on pins 4, 5, 7 or 8 behaves exactly like this: it links, it passes web pages, and it tests at about 94 Mbps.
Expected result at each link speed
| Negotiated link | Pairs used | Healthy test result |
|---|---|---|
| 10 Mbps | 2 | about 9 Mbps |
| 100 Mbps | 2 | about 94–95 Mbps |
| 1 Gbps | 4 | about 930–941 Mbps |
The 94 Mbps figure is protocol overhead on a 100 Mbps link; see why gigabit tests at 940 for the arithmetic. If your result sits right at 94, the link negotiated 100 Mbps; it is not a mystery ISP limit.
Diagnosis in order
- Read the link speed from the device. Windows: Settings, Network, Ethernet properties show "Link speed". macOS: System Settings, Network, Ethernet, Details, Hardware. Many routers also show per-port speed or an amber LED for 100 Mbps.
- Swap the patch cable for a short known-good Cat 5e or Cat 6 cable. Very old Cat 5 or flat cables, and any cable crimped by hand, are the first suspects.
- Change the port on the router or switch, and try a different device on the same cable. A single port stuck at 100 Mbps points at the port; a device that is slow on every port points at the device or its driver.
- Check the middle of the path. Wall sockets, patch panels and cheap couplers each have their own connections; a 100 Mbps-only switch or powerline adapter in the chain caps the whole path.
- Retest after a reboot of both ends. Autonegotiation runs when the link comes up, so a link that fell to 100 Mbps after a glitch stays there until it renegotiates.
A worked check with a before-and-after test
Suppose a desktop on a 500 Mbps plan tests at 93 Mbps by cable, while a phone on Wi-Fi in the same room tests at 380 Mbps. The Wi-Fi result proves the plan and router are fine, and 93 Mbps is the 100 Mbps overhead ceiling, so the wired link is the problem. Swapping only the patch cable and retesting is the cheapest first step; if the result jumps to about 470 Mbps, the old cable had a bad pair. If it stays at 93 Mbps, move to the port and device checks above and change one thing at a time so you know which one fixed it.
When 100 Mbps is the right answer
Some devices only have 100 Mbps ports, including older smart TVs, set-top boxes and some printers. That is enough for a single 4K stream; see our streaming bandwidth calculator. It is not enough to measure a plan above 100 Mbps, so test from a laptop with a gigabit port instead.
Related
Router placement · Why results change · Test Wi-Fi speed accurately · Methodology · Mbps to MB/s converter