Defense in depth is “is a strategy that leverages multiple security measures to protect an organization's assets. The thinking is that if one line of defense is compromised, additional layers exist as a backup to ensure that threats are stopped along the way” (Fortinet, n.d.). This can also be called redundancy, which means there are several tools in place with the same goal that will pick up should one fail. This is done across the field of technology, and not just security. Though in security, these mechanisms are arguably the most important, as a security breach can bring the rest down in a matter of minutes or hours. Think of defense in depth like a neighborhood, where we can make it safer and safer. We begin with the house, which we make safer with locks on the doors (or passwords on the host) then a firewall (or an alarm system on the doors and windows). The house already feels safer, but there are still risks. So we add a fence around the house, which would be closest to the antivirus as it will add another layer of perimeter security. Now, the home is secure, but it would be much safer if the homes on your street all had lights on the street and garages (a network monitoring program), so it is easy to see people in the street and identify possible threats. Well, it would be even more safe if you added a large fence around the community, with one gate at the front. Think of this like a subnet, where you need to authenticate at the gate to be granted access (multi-factor authentication like RADIUS servers). So the rest of the area around you could be accessed, but even if it were you would be safe. Now put a guard at the gate that looks for suspicious activity, (like an Intrusion Detection and Prevention System) and your home is now extremely safe. We went from an exposed home on a random road (or PC on a network), to a very secure and safe neighborhood (or device) just by adding layers of security on top of one another. 



Fortinet. (n.d.). What is defense in depth? defined and explained. https://www.fortinet.com/resources/cyberglossary/defense-in-depth#:~:text=Defense%20in%20depth%20is%20a,are%20stopped%20along%20the%20way.