Don’t Use “Guest” WiFi Hotspots for Local Multiplayer Minecraft

After having heard of (but not paid much attention to) the game “Minecraft”, I got it for
10-year-old Anthony and me to play together, and found it to be delightful
fun. The gameplay is completely unstructured (you explore, mine for
resources, and build stuff), though how things work is a bit opaque at
first until you figure out how to build stuff. Anthony did so quickly, so
he kindly explained it to me.

However, I ran into trouble setting up the WiFi link to play together, and felt like an idiot once I realized what the problem was. I thought I'd
share it here for the search engines, in case others had problems setting up multiplayer Minecraft over local WiFi.

The standard instructions are simple:

Ensure both devices are on the same WiFi hotspot.
On one device, open the world you want to play; ensure that the “local server multiplayer” option is enabled. Start playing.
On the other device(s), press the [PLAY] button to bring up the list of worlds.
See the WiFi world shared by the first device, select it, and go build things together.

For the life of me I could not get this to work... no shared world appeared in the last step... until I realized that “guest” WiFi
networks (such as created by an Apple Time Capsule or Time Machine, and
probably many other home WiFi routers) explicitly isolate each client from
all others, so even if both devices are connected to the same hotspot, they
can not talk to each other.

I'm sure it's the same for most public WiFi hotspots.

So don't use “guest” or public WiFi hotspots for local
Minecraft multiplayer (or any local multiplayer game, for that matter), and
you'll not waste an hour of your life pulling your hair out.

Oh, it's getting dark and I hear a zombie coming, so I gotta go...