ASUS Router? Update your firmware (update: general net issues after Windows update)
After running the latest batch of Windows updates, one computer after another stopped connecting to the internet and even to local networking. The Windows update was clearly the problem. They couldn't even ping anything else on the network. Some sort of tcp/ip stack update by the looks of things.
As it turns out, I have an upstairs WiFi router which is connected into the downstairs ASUS modem/WiFi router and connecting to that upstairs WiFi router worked. Oddly, machines thus connected showed as "static" ip address instead of "DHCP", even though they had received their ip address from DHCP.
If it hadn't been for the second WiFi router, I'd've not had any internet connection at all.
TL;DR: Update your ASUS WiFi router's firmware, and everything will work again as normal. Again, must be a tcp/ip issue, but I can't be arsed to track it down at the moment.
ON EDIT: And now the upstairs router doesn't allow access to the local network and half of the WiFi routers in the neighbourhood have disappeared off the network. Problem must be more general than just ASUS routers...