My networking know-how is limited, though usually adequate for my needs. A friend had a problem that neither of us could solve but a Vodafone magician did, with a simple suggestion. The problem was that his Internet had died and we couldn't figure out why. His router also feeds his tenants and when it was upgraded he just plugged everyone into the ports on the back, as he had done with the previous one. This worked fine for months. Then it didn't.

The VF magician asked him to look at the ports to see if any was marked WAN. One was, and as soon as he unplugged it, everything started working again.

I have a vague understanding of WAN vs LAN from the early days, but this experience has made me wonder. Why did everything work with this configuration for several months, and then suddenly stop? Why did unplugging the WAN port fix all the others? What is the practical difference between LAN and WAN in a modern router like this? I am just curious, if anyone would care to explain.