When I stepped into the field of networking, everything was static and security was based on perimeter-level firewalling. It was common to have two perimeter-based firewalls; internal and external to the wide area network (WAN). Such layout was good enough in those days.
I remember the time when connected devices were corporate-owned. Everything was hard-wired and I used to define the access control policies on a port-by-port and VLAN-by-VLAN basis. There were numerous manual end-to-end policy configurations, which were not only time consuming but also error-prone.
There was a complete lack of visibility and global policy throughout the network and every morning, I relied on the multi router traffic grapher (MRTG) to manual inspect the traffic spikes indicating variations from baselines. Once something was plugged in, it was “there for life”. Have you ever heard of the 20-year-old PC that no one knows where it is but it still replies to ping? In contrast, we now live in an entirely different world. The perimeter has dissolved, resulting in perimeter-level firewalling alone to be insufficient.