The cable for Ethernet can be hot plugged. However, there are also different qualities of cable, both regarding the twisted pairs of wires within the cable as well as the shielding, or absence thereof, surrounding the wires.

hazel,

Ethernet connections are hot-pluggable.

Dumb question maybe: would anyone with more than one PC on a home LAN ever intentionally have the router off, except briefly to reset it or test something using it? It seems like there is no functional point hunting for an answer to hazel's question. If I had to guess, my money is on PXE-related, and disabling PXE in BIOS should be a solution. The only time PXE is enabled here is when I've done a BIOS reset, or the CMOS battery died, and I haven't changed it yet.

I switch everything off when I'm not using it. I switch on the router and the desktop computer together every morning and switch them both off again in the evening. Is that unusual behaviour? Quite apart from everything else, my router seems to lose its internet connection if a long time passes with no traffic on it. The lights still flash but nothing comes through. Then I have to power-cycle it. So it would actually be disadvantageous as well as a waste of electricity if I left it on overnight.

I have a laptop too, but I seldom use it.

As to why I'm pursuing this, the explanation is simple curiosity. I want to know why.

Yesterday I tried booting with the cable connected to the computer but not to the router. That worked. So I think michaelk's SWAG is pretty close to the truth. It's the router that is doing something odd to the cable when it is switched off. I have now switched to a different port on the router and will try the same tricks with that one.

Maybe something with the cable, do you have another known good cable you can swap out with?

I would expect a BIOS message if it was a PXE failed error. Just to satisfy mrmazda's curiosity and mine now what is the boot order?

I think most people leave their routers on full time. I know I do. I have backups scheduled for the middle of the night or very early AM when I won't be using the computer, so I leave the computer on as well. I also have a security camera and garage door opener connected to the router via wifi. I could do without the opener, but the camera is always on. TBH, you're the first person I've ever heard of who turns off their router, ever, except for temporary moves or troubleshooting. They don't draw much power, and most people don't care anyway. So yes, I would consider that unusual, but not something that should cause any problems. I'm also curious as to why the computer doesn't boot if connected to an unpowered router. That does not compute, to coin a tired phrase. Another thought that occurs is to test if the same happens if connected to the router, but the power cable is not connected. On so many devices, just turning the power switch off does not actually turn them completely off. Does this behavior persist with no possible power to the router?

That's a lot of new things to try out! As far as the mechanism is concerned, michaelk suggested one that sounds reasonable to me: that the router port I was using has such a high impedance when off that it turns the cable into an aerial and the aerial picks up something that bugs the UEFI program. So we'll see today if using a different port makes any difference. And yes, I do have another cable which I can test. I haven't yet tried with the router unplugged.

If you read my last post again, you will see why leaving the router permanently on is a nono. It has some kind of glitch that makes it lose contact with the net if there is no activity for an hour or so.

Perhaps there is a connection between the symptoms?

Not suppose to be, but I've never damaged anything doing it.

So your really believe enterprises with mega ethernet switches are by default required to power down their switches and kill networking for possibly hundreds or even thousands of employees to simply connect a new device.?? That would be asinine to be required to do that.

Cat 5 & 6 ethernet is hot-pluggable by design. (Unlike the earlier coax ethernet which was one continuous daisy chain and had to be terminated on both ends of the chain.)

Now, as to Hazel's question as to why. I have never seen a system do as she describes, but there is one thing that may be part. An open ended cable does not see the other end connected so it is "dead ended" and the system ignores it. OTOH a cable that is connected on both ends is seen as "connected" and is treated differently. Her's sees the router connected but no communication so it seems to hang expecting it to reply to communication attempts.

@hazel, Do you by chance use DHCP to get your address for networking? If so that might contribute to the issue since the cable is connected but gets no reply and the dhcp request hangs waiting for an answer.

That behavior to me implies the router might be in the early stages of failure, as does the interference with the PC booting (which should not happen). The PC should see a connected device that is powered down as "open" and ignore it the same as a disconnected cable. Depending upon the age of the router it might be worth replacing the router before a catastrophic failure also takes out connected ethernet cards in the PC.

If the router is provided by the ISP I would place a repair ticket based on the failure to maintain a connection, and hope for a new replacement. If it's your own property, I would start preparing to replace it. Is this a combination modem/router, aka gateway, or a separate router connected to a modem?

With hot plugging there are two parts, 1. the physical connection and 2. the operating system.

Ethernet is a low voltage differential signal, no ground and typically no power (non POE). The signals are galvanic isolated from ground but it is possible to damage the ports with static electricity. The hardware detects if there is link.

Most kernels/operating systems these days are configured for ethernet hot plugging so it can detect when a cable is connected and disconnected and automaticall to WiFi if so equipped.

Interesting - something I thought of here is: I remember some older motherboards having 'cable sense' for their Ethernet ('is anything plugged in' basically), and perhaps your BIOS is doing this as well, and detecting 'no media' and hanging on that. See if there's a setting related to that in the BIOS options.