The layered model, TCP versus UDP, congestion control, routing, and what happens when you load a URL.
4 questions in CS Fundamentals.
Six steps in order: the browser checks its cache, DNS resolution turns the hostname into an IP address, a TCP handshake opens a connection, a TLS handshake authenticates the server and agrees keys, an HTTP request fetches the page, and the browser renders it. Panels push on DNS, the handshakes and certificates.
TCP guarantees an ordered byte stream and pays for it with head-of-line blocking and retransmissions that arrive too late to be useful. UDP is right when a late packet is worthless, when you need multicast, or when you intend to build your own reliability with different trade-offs.
A TTL is an upper bound on how long a resolver may cache a record, not a promise about when clients will notice a change. Lowering it only takes effect after one old TTL has elapsed, negative answers are cached under a different rule, and pooled connections keep using an address they resolved hours ago.
The browser checks its caches, resolves the hostname through browser, OS and resolver caches, opens a TCP connection, completes a TLS handshake, sends the request, then parses the streamed response. QUIC fuses transport and crypto setup into one round trip and stops one lost packet stalling every stream.