"Capital I" internet. Love this aesthetic.

It is a proper noun, in contrast to the other internets. If there is a guy called Guy you would not call him guy.

It still annoys me whenever the public internet known as the Internet is misspelled with a lower-case i.

Hmm. Trying to remember if places I've worked at talked about "The Intranet" or "the intranet." I guess you could use "Intranet" as a proper noun if you're talking about the one in use at your company but then "intranet" if you're talking about the concept of an internet inside your company's firewall that's not connected to the Internet.

There are millions of intranets but only one Internet, that's the whole point.

Right, but there are scads of "internets" (with the lower-case 'i'), which is exactly the point.

Yep, we write "the Internet". When was the last time someone wrote "an internet"? :)

Three responses ago.

But yes, capitalized "Internet" refers to the "Connected Internet," of which there is only one. The first rule of SIPRnet is no one talks about SIPRnet. But if we did talk about it, the comment would likely be that it is like "the Internet" but not "the Internet."

If you spend time in academia or academia-adjacent industrial research, you sometimes hear "an internet" (always with lower case to signal it's a common noun) to describe a network of networks. But you are right... that use is not growing and if anything shrinking.

Maybe some aliens out there have their own internet. But then is the internet just the sum of all intranets and thus in a way just the top-level intranet?

  data Intranet = Intranet [Intranet]

They were practically daring Microsoft to create VBScript, which it did.

And later it created JScript. Embrace and extend... this is the way.