The original writeup https://fabiensanglard.net/quake_chunnel/index.html

The mid-90s was arguably the period in PC history with the most whiplash-inducing changes. The arrival of 3D acceleration cards, the transition from plain-textbox DOS to fancy Windows 95, and the advent of the Internet all happened at once. This rapidly shifting landscape posed quite the challenge for game developers, as they had to consider writing their games for DOS, Windows 95, or both.

In an exceedingly detailed writeup, Fabien Sanglard explains how the OG Quake got its support for TCP/IP and was arguably the only game that used the same executable with native support for both operating systems.