>>The problem described below has been solved, but I don't quite understand it.
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>>This morning, I couldn't connect to the Internet from my computer. I used my children's computer for some emergencies.
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>>When doing a ping, from my computer, to the default gateway (the ADSL router), about half of the tries were successful.
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>>The other computer worked perfectly, so the problem didn't seem to be the ADSL router or switch.
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>>I contacted the person who does tech support at our local Cisco Academy, and he said, more or less: Well, you might try buying a new network card. After all, they are not that expensive. Or perhaps demagnetize the network card.
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>>Following his instructions, I unplugged the computer power cable, and touched the metal contacts with a metal tool that had an insulated handle. (He said "screwdriver", bud I didn't find one at that moment.) That was the connection for the power source, I didn't do anything with the network card.
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>>The whole idea is preposterous, except for one detail: it actually worked. I had seen it work before, on one of the computers at the local Cisco Academy (but I hadn't remembered the details).
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>>Important note: Careful with this - I understand the power supply can have dangerous voltages, even with the power cord unplugged.
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>Had you already tried power off/power on on the machine? I'd guess that that would have cleared it if the problem was just something flakey in the NIC..

Yes, I had previously turned the computer off and on again. The same for the networking equipment (switch, ADSL router) - although the problem didn't seem to be related with those (the other computer, connected to the same networking equipment, worked fine).

Which reminds me that I had also suspected the network cable (more specifically, a bad connection, either to the computer or to the switch). But the result shows that (apparently) that wasn't the problem.