Consider that when Trumpet Winsock was a third-party IP add-on for Windows 3.1, classic MacOS already had IP support built-in.

Ironically around that time Microsoft was selling Xenix (https://en.wikipedia.org/wiki/Xenix) which did support it.

Originally Apple's MacTCP was actually a product they sold separately, it wasn't bundled into the OS until 1994. Although it was usually easy to get a hold of a copy since universities and such had site licenses, and ISPs often had redistribution licenses

Ah, it was introduced in System 7.5 which, perhaps coincidentally, was the first version of MacOS I had when I needed to connect it over TCP/IP.