Written 6 Sep 2026. This is a how-to, not a product card.

When the cable is plugged in but there's no link

Ethernet is seated, but SSH still hangs or the computer never gets a house address. Check both ends, LAN vs WAN, link lights, then ip link before you buy a NIC.

Want the calm "plug it in on purpose" note first? That is a cable beats Wi-Fi for the computer. This page is the failure case: the click is in both jacks, and the house still acts like the cable is dead. Don't buy a USB adapter tonight. Don't open port 22 on the router to "test."

Skip until you can touch the computer

You need eyes on the ports, or a keyboard and screen on the computer, or a working Wi-Fi login so you can run commands. If SSH already works on the house radio and you only care that the cable is dark, stay on Wi-Fi for now: put the old PC on house Wi-Fi. If nothing answers on any path, prove the box is awake: is it still on?

What this is

A cable "link" means the computer and the router (or switch) both see each other at the jack. No link means no house address on that plug, so SSH and Pi-hole on that path stay dead. Most nights it is a loose click, the WAN port by mistake, a dead patch cable, or the cable interface still DOWN while Wi-Fi still holds the only address. The walk below is physical first, then lights, then ip on the computer, then the usual traps.

Step 1: both ends, LAN not WAN

Unplug and reseat both ends until you feel the click. One end is the computer. The other end is a LAN port on the house router (or a switch that already talks to that router). Do not use the WAN / Internet / globe port. That port faces the street. A house computer on WAN often gets no useful address, or fights the modem. If the cable runs through a wall jack, treat that jack as one more click to reseat, or try a short known-good cable straight to the router for one proof.

Step 2: try a different cable and a different LAN port

Swap in another Ethernet cable from the drawer. Cat5 is fine. Move to a different yellow or numbered LAN port on the router. Skip the one labeled WAN. If you have a small switch in between, power-cycle that switch once, then reseat both ends again. One dead cable ruins a whole evening of software guesses.

Step 3: look for link lights

On many OptiPlex and mini PCs, the ethernet jack has a small LED. On many routers, each LAN port has a light when something is linked. Dark on both ends after a firm click usually means no carrier: wrong port, dead cable, or the computer's port is off or broken. A light on the router and dark on the computer (or the reverse) still points at the cable or that one jack. Leave fancy speed colors for later. You want any link light first.

Step 4: see the cable interface on the computer

If Wi-Fi SSH still works, use that. Otherwise plug in a keyboard and monitor. Print short link status:

ip -br link

Then addresses:

ip -br addr

The cable name is often eth0, or it starts with enp or eno. The radio is often wlan0, or it starts with wlp. You want the cable line UP, not DOWN, and not stuck with NO-CARRIER in the longer view:

ip link

NO-CARRIER means the OS sees the port, but nothing is linked on the wire. Go back to Steps 1–3. Missing cable name entirely can mean a USB dongle that never got seated, a broken onboard port, or a driver the live install never loaded. Confirm the physical jack before you shop.

Step 5: bring the cable up and ask for an address

If the cable line is DOWN, turn it on (swap in your real name from Step 4):

sudo ip link set eth0 up

Ask DHCP again on that plug if your install uses the usual client:

sudo dhclient -v eth0

Some Ubuntu Server installs use netplan and systemd-networkd instead of a hand dhclient. If dhclient is missing, check whether the cable already has a 192.168… or 10… on ip -br addr after the link is UP. No address with link UP often means the router DHCP pool is exhausted, the computer was set to a wrong static number, or you are still on the WAN port. Router-side calm reservation later: keep the computer on the same address on common routers. Hand static only when the router will not reserve: give the computer a fixed address when the router won't.

Step 6: prove the cable path, then park Wi-Fi

Once the cable has a house address, find that number the usual way: find it on the house network. From the laptop on house Wi-Fi:

ssh you@192.168.x.x

Use the cable address, not an old Wi-Fi number you wrote down last week. When SSH works on the cable, turn the radio off as a check (use your real Wi-Fi name):

sudo ip link set wlan0 down

SSH should still work. If it dies, the cable never took the job. Reseat again. When the cable is boring, leave Wi-Fi off across reboots: keep Wi-Fi off after a reboot.

Step 7: match what you see

Match the symptom before you buy anything:

What this is not

This is not a reason to buy a 2.5G switch tonight. It is not a new motherboard because one patch cable was bad. It is not forwarding SSH to the internet to "bypass" a dark LAN port. It is not turning off the house firewall on your laptop. Stay on a known-good cable and a LAN port until link is boring.

What next

When the cable holds SSH, keep Wi-Fi from coming back: keep Wi-Fi off after a reboot. Optional speed check later: see the cable speed. Still hunting the number after a reboot: find it on the house network. SSH still odd with a good link: when SSH won't connect. No ethernet port in reach? put the old PC on house Wi-Fi. The calm plug-in note stays: a cable beats Wi-Fi for the computer.

Skip

• Skip this if ip -br link already shows the cable UP with a house address and SSH works on that number.
• Skip a new USB NIC until a second cable and a second LAN port both fail.
• Skip a 2.5G switch, mesh kit, or NAS for a dark link light.
• Skip forwarding port 22 on the router to "test" the cable.
• Skip buying Cat6 because Cat5 was in the drawer.