OSI layers: 1.Physical (bits, cables), 2.Data Link (frames, MAC, switches), 3.Network (packets, IP, routers), 4.Transport (segments, TCP/UDP), 5.Session, 6.Presentation (encryption, encoding), 7.Application (HTTP, DNS, SMTP).
Mnemonic: "Please Do Not Throw Sausage Pizza Away" (bottom-up) or "All People Seem To Need Data Processing" (top-down). Troubleshoot bottom-up. Firewalls at L3/L4. WAF at L7. Switch at L2. Router at L3. Know which protocol/device operates at which layer.