A short history of segmentation
Let’s start this with quoting wikipedia on what network segmentation is according to a encyclopedia.
Network segmentation in computer networking is the act or practice of splitting a computer network into subnetworks, each being a network segment. Advantages of such splitting are primarily for boosting performance and improving security.
How did this start? Some of the readers might actually be old enough to remember how local area networks started out in the early 90s.