After rewiring my network, the whole house was stuck at about 84 Mbps. The speed test numbers pointed to a link running at 100 Mbps somewhere. Testing one piece at a time found a damaged network cable. The same day, a TV that couldn't stream from my Plex server turned out to be plugged in on the wrong side of the firewall.
Result: a new cable brought the connection back to full speed, and the TV now streams from Plex on the local network.
The layout
The house's fibre box (ONT) has only one active port. An unmanaged switch on that port feeds two things: the household router, which I leave alone, and my firewall, which runs the lab network. The ISP gives each its own public IP.
Fibre ONT
└─ Unmanaged switch
├─ Household router
└─ Firewall (OPNsense)
└─ Lab switch ── lab devices1. Reading the speed test
Speed tests showed 84 Mbps down and 81 up, and it affected the whole house.
Two things stood out:
- Download and upload were almost equal. A slow connection from the ISP doesn't usually look that even.
- Both were just under 100. A 100 Mbps network link can't deliver the full 100 once overhead is taken out, so a result in the 80s and low 90s is what a 100 Mbps link looks like.
Since the whole house was affected, I started with what everything shares: the switches and cables near the ONT.
2. One change at a time
My first move was to unplug the firewall. That changed two things at once, the firewall and its cables, so the result couldn't tell me which one mattered.
I started over and tested one switch and one link at a time, with the firewall unplugged. The lab switch's port lights gave the answer: on the link to the unmanaged switch, the port showed one light (100 Mbps) instead of two (gigabit).
3. The cause: a damaged cable
Gigabit Ethernet uses all four wire pairs in a cable. 100 Mbps only needs two. If a cable has a damaged pair, the devices on each end don't report an error. They quietly agree on 100 Mbps instead, and everything still works, just slower.
A new cable brought the link back to gigabit and the speed tests back to full speed.
4. The TV that couldn't stream
The same day, a TV I'd wired in gave a "not fast enough" playback error when streaming from my Plex server on the lab network.
The TV was plugged into the unmanaged switch at the ONT. That put it directly on the internet, outside both the household network and the lab. To Plex, the TV looked like a remote device, so instead of streaming over the local network it fell back to Plex's relay, which is heavily speed-limited.
| Step | What I found | Fix |
|---|---|---|
| Where it was plugged in | The switch in front of the firewall, so it was outside the lab network | Moved it behind the firewall, onto the same network as the Plex server |
| Its wired port | 100 Mbps only | Switched it to 5 GHz Wi-Fi, which is faster than its wired port |
A switch in front of a firewall is outside the network, even when it's sitting in the same rack.
What I learned
- The numbers can point to the cause. Download and upload both just under 100 meant a 100 Mbps link, not a slow ISP.
- Change one thing at a time. My first test changed two things and told me nothing.
- Check the port lights. They showed the link speed right away, with no tools.
- A bad cable doesn't always fail. It can drop to 100 Mbps and keep working, which makes it easy to miss.
- Know which side of the firewall a device is on. The TV was plugged into the right rack but the wrong network.