Don't memorise all seven. Know the rough idea:
- Layer 3 (Network) — IP routing. Routers, IP addresses, ICMP.
- Layer 4 (Transport) — TCP/UDP. Ports, connections, retransmissions.
- Layer 7 (Application) — HTTP, DNS, TLS. Load balancers and CDNs operate here.
When someone says "layer 4 load balancer," they mean it routes by IP + port without understanding HTTP. "Layer 7 load balancer" routes by hostname, path, headers — it understands the application protocol.