HTTP, DNS, TLS, browsers, and how the web actually moves data.
What actually happens between typing a hostname and getting an IP address back — the lookup chain, caching layers, and why DNS is so often the first suspect in an outage.
The HTTP status codes that actually show up in day-to-day support and operations work, what each one really means, and which ones point at the client vs. the server.
What actually happens during a TLS handshake, why certificate expiry breaks everything at once, and how to read the handshake yourself with openssl and a browser.