## Defense in Depth (Layered Security)

Do not rely on one wall. If the wall fails, you lose. Use multiple layers.
The "Onion" approach.

### The Layers
1.  **Policies/User**: Training, Strong Passwords.
2.  **Physical**: Locks, Cameras, Guards.
3.  **Perimeter**: Firewall, DMZ.
4.  **Network**: VLANs, NAC (Network Access Control).
5.  **Host**: Antivirus, EDR, Patching.
6.  **Application**: Secure Code, Input Validation.
7.  **Data**: Encryption, ACLs.

### Example
A hacker wants your database.
1.  **Firewall** blocks their port scan. (Layer 3).
2.  They Phish a user. **Email Filter** misses it.
3.  User clicks link. **EDR** detects the malware download and blocks it. (Layer 5).
**Defense in Depth worked**.