[iRomain]

It is a valid domain in the technical DNS sense. But practically speaking, each TLD has "ns" entries in the root zone database, not "A", "CNAME", etc. entries. This is just a guy who registered www under a TLD that's it.

[LeoPanthera]

This has nothing to do with the "www" subdomain. I assure you that the unix "host" tool is not trying to guess subdomains if the lookup fails:

  $ host "ai."
  ai has address 209.59.119.34
  ai mail is handled by 10 mail.offshore.ai.

[iRomain]

I'm not saying "ai" is not a valid domain. I'm just guessing why people get to a website when they just type "ai".

I'm on my phone now, if you search for the ai entry in the root zone file (https://www.iana.org/domains/root/files) you should find that IP returned by your host command.

Can you try curling "ai" and see if the content returned (if any) is the same as the one from "www.ai"?

[LeoPanthera]

They are the same, but that's not really the point. www has been registered in addition to the entries for the root TLD. You said "the internals of the browser that automatically adds a www in front" - that's irrelevant. host isn't doing that, curl isn't doing that.

[iRomain]

You are right, they added a A record in the root zone file. I didn't know it was allowed.

https://news.ycombinator.com/item?id=31690379

[5ESS]

Did the owners of .com do the same thing?

Because http://com. Works as well.

[captn3m0]

There's a bunch of TLDs with A records, even Google used to point .dev to 127.0.0.53 for a while.

[iRomain]

You are right I didn't know it was allowed