One day my MIL's Macintosh stopped being able to connect to the Internet over its internal ethernet, which was directly connected to the cable modem.

They called Comcast a bunch of times to no avail, so they sent someone out to check it. He had no idea what was wrong, so I said "Let's connect your laptop to the Mac with an Ethernet cable just to make sure the Ethernet works."

Dude looked at me like I had two heads. "It doesn't work like that."

I proceeded to grab a patch cable, hook them together, and mount the Mac's public shares on the Windows machine, thus proving the Ethernet worked on both systems.

Turns out Comcast had changed the MTUs on the modems one night, which made the Mac not work for some reason. But getting a cheap router and putting it between solved the problem.