Except that also on the study extension is the router. Argh! No internet!
A quick rearrangement, and the router is now in the hallway. Thus, wireless is back, meaning that the MacBook is usable again. On the other hand, the router is now out of RJ45 range, unless we want to rig our hallway with death-traps, so the desktop is still an oversized, dusty paperweight.
Tomorrow's mission: get another extension cable.