"Explain to me how the internet works."
I believe it has all the characteristics of a good interview question.
It's an open question
There's no right or wrong. The candidate can tell whatever he/she wants, it's about how they explain it and how comfortable they feel doing so. It will inevitably be followed by the question "could you be more specific?" to get a feel of where you're going with the question.
Don't answer. Let them figure it out as they try to explain.
It's for network engineers
For network positions it's a great way to talk about routing, IP, VLANs, BGP, networks, OSPF, AS-numbers, ...
It's a perfect entry to discuss anything deeply technical afterwards. If during the answer they show a thorough understanding of one the layers, dive in. Find out what they're passionate about, because at the end of the day, it's that passion that drives us.
It's for sysadmins
As a sysadmin you can explain how servers work, the application layers, web- and database servers, caching, DNS, TCP/IP, ...
This is a good time to show how you fully understand the server stack from top to bottom. This could even be a good time to inject a personal hero-story of an epic debug mission that succeeded for you, by analysing the server stack entirely.
It's for webdevelopers & marketeers
Understanding the basics of "the internet" is crucial for anyone involved in the web. Therefore being able to explain, from a high-level, how it works puts the candidate in a better position in general.
If something ever goes wrong, you'll know you have a candidate that not only knows its own field, but has enough knowledge to think outside the box and look at the bigger picture to troubleshoot and diagnose the problem.