FAQ
TL;DR: If your PC negotiates 100 Mb/s, real downloads cap near 94–95 Mb/s; “Yes, it’s the maximum speed.” [Elektroda, sanfran, post #16930541]
Why it matters: This FAQ helps home users diagnose why a gigabit-capable setup only delivers ~95 Mb/s and how to reach true 1 Gb/s.
Quick Facts
- A 100 Mb/s Ethernet link typically speed‑tests at ~94–95 Mb/s due to overhead. [Elektroda, Jozef2038, post #16930528]
- Link speed is the lowest common denominator of NIC, switch/router, cable, and settings. [Elektroda, JacekCz, post #16930551]
- Damaged/poor cables can force 1 Gb/s hardware to fall back to 100 Mb/s; replacing the cable restored 1 Gb/s. [Elektroda, Jozef2038, post #16937741]
- Check that 1 Gbit is enabled in your adapter/router configuration. [Elektroda, alvaro99, post #16932979]
- Flexible end-cables are sold up to ~15 m; older guidance suggested ~5 m for patch leads. [Elektroda, JacekCz, post #16937802]
Why does my 120 Mb/s plan only hit ~95 Mb/s on speed tests?
Your PC is linking at Fast Ethernet (100 Mb/s). With protocol overhead, measured throughput lands near 94–95 Mb/s. “Yes, it’s the maximum speed.” To exceed ~95 Mb/s, the link must negotiate 1 Gb/s end‑to‑end (NIC, router/ONT, and cable). [Elektroda, sanfran, post #16930541]
How do I check and enable 1 Gbit on my Windows NIC?
Open Device Manager → Network adapters → your NIC → Advanced. Ensure Speed & Duplex is on Auto Negotiation or 1.0 Gbps Full Duplex. Also confirm your router/ONT LAN port supports 1 Gbit. If 1 Gbit is disabled anywhere, you’ll cap at 100 Mb/s. [Elektroda, alvaro99, post #16932979]
Can a bad Ethernet cable force 100 Mb/s instead of 1 Gb/s?
Yes. A damaged or marginal cable can drop a gigabit link to 100 Mb/s. Replacing the suspect cable restored a 1 Gb/s link, and tests jumped from 95 Mb/s to 130–140 Mb/s (initial burst ~400 Mb/s). [Elektroda, Jozef2038, post #16937741]
Which should I buy: Cat5e or Cat6 for a short home run?
Choose a quality, brand‑name cable and avoid sloppy terminations. The thread focuses on using good cable rather than crimped ends, not on marketing labels. Plan proper terminations for reliability. [Elektroda, JacekCz, post #16937759]
What exactly is the "fiber optic center" box from my ISP?
Provide the exact model labels. Helpers asked for device models because behavior varies by ONT/router. Knowing the model clarifies port speeds and service VLANs. [Elektroda, Nagus, post #16932184]
My speed test starts near 400 Mb/s then settles at 130–140 Mb/s—normal?
Short bursts can hit cached or TCP window peaks, then stabilize to the sustained rate. After fixing the cable, results stabilized near 130–140 Mb/s with an initial spike around 400 Mb/s. [Elektroda, Jozef2038, post #16937741]
Quick 3‑step: how do I diagnose a 100 Mb/s link on gigabit hardware?
- Bypass in‑wall runs: move the PC next to the ONT/router and use a known‑good short patch.
- Check NIC settings: set Auto or 1.0 Gbps Full Duplex.
- Swap the patch cable; if link LED or status jumps to 1 Gb/s, replace the old run. [Elektroda, Jozef2038, post #16930685]
What’s the difference between Mb/s and MB/s when comparing speeds?
ISPs quote megabits per second (Mb/s). File managers show megabytes per second (MB/s). Divide Mb/s by 8 to estimate MB/s. Example: 120 Mb/s ≈ 15 MB/s. Mixing units leads to confusion about expected download rates. [Elektroda, wesoł2008, post #16930562]
Can flaky cabling make gigabit unstable even if it negotiates 1 Gb/s?
Yes. Poor cabling can cause retransmissions and drops. Many prefer forcing 100 Mb/s on weak runs for stability over an unreliable 1 Gb/s link. “I prefer to have a stable 100.” [Elektroda, JacekCz, post #16930551]
Do TV set‑top services on other ports slow my PC’s internet?
Not in this case. The user saw the same ~95 Mb/s regardless of TV or router activity. The bottleneck was the Ethernet link and cabling, not the active TV ports. [Elektroda, Jozef2038, post #16930652]
Should I use shielded Ethernet for a 10 m room‑to‑room run?
Focus first on build quality and proper termination. One expert emphasized choosing rigid cable terminated in sockets over ad‑hoc crimped plugs for reliability. Good terminations beat questionable shielding in short runs. [Elektroda, JacekCz, post #16937759]
How long can patch cables be, and does length affect performance?
Patch leads are commonly sold up to about 15 m. Older guidance suggested around 5 m for flexible end‑cables in structured cabling. Excessive length and poor quality can impair gigabit stability. [Elektroda, JacekCz, post #16937802]
What if my Intel 82579LM says 1 Gb/s but Windows shows 100 Mb/s?
The NIC supports 1 Gb/s, but the link negotiates to the slowest element. Check the NIC setting, router/ONT port speed, and especially the cable. A damaged 5 m run forced 100 Mb/s here. [Elektroda, Jozef2038, post #16930652]
but it drops to 130-140 and that's the speed. On an old cable, each result is 95Mbps. And the most important thing on the new cable is 1Gb / s 
