What networking fundamentals should every engineer know?

• OSI / TCP-IP model — link, network (IP), transport (TCP/UDP), application (HTTP, DNS, SMTP)
• DNS translates names to IP addresses; TTLs, caching and failover records affect how fast changes propagate
• TCP is reliable and ordered; UDP is fast and connectionless (QUIC/HTTP-3 builds reliability on UDP)
• HTTP/HTTPS — methods, status codes, headers, caching and cookies; TLS provides encryption, authentication and integrity
• Latency vs bandwidth — round trips usually dominate web performance; reduce them with CDNs, keep-alive and compression
• Subnets, firewalls, VPNs, NAT — segment networks, restrict traffic by rule and protect remote access
• Real-time channels: WebSockets and server-sent events for live updates
• Troubleshooting toolkit: ping, traceroute, dig/nslookup, curl, packet captures

Understanding these makes outages, slow pages and security issues diagnosable instead of mysterious.