Firewall designs have evolved over time from simple flat designs to complex layer designs for defense in-depth like DMZ.
Reference: CISSP Study Guide 3rd Edition - Eric Conrad
https://www.amazon.com/gp/aw/d/0128024372/ref=dp_ob_neva_mobile
https://resources.infosecinstitute.com/topic/firewall-types-and-architecture/#:~:text=There%20are%20four%20common%20architectural,firewalls%20and%20screened%20subnet%20firewalls.
