Your question is Triage a Layered Network Outage. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You support a hybrid environment with user endpoints, switches, routers, load balancers, and internet-facing applications. Users report that some internal services are reachable by IP but not by hostname, while an external web application intermittently fails TLS handshakes and health checks. At the same time, Tenable Nessus and Tenable Vulnerability Management scans show assets are online, but some authenticated checks have started failing. You need to explain the issue clearly and drive troubleshooting across the right network layers.
Explain the OSI model and why it matters when diagnosing networking and security problems in this environment. Then describe how you would use it to narrow down these symptoms, identify likely failure points, and verify that service is fully restored.