Network Fundamentals

TCP/IP Basics

TCP guarantees delivery with retransmission — use for reliability (HTTP, SSH, databases)
UDP is fire-and-forget — use for speed when loss is acceptable (video, gaming, DNS queries)
Port numbers: 0-1023 privileged (need root), 1024-65535 available — common services have well-known ports
Ephemeral ports for client connections — OS assigns randomly from high range

DNS resolution is cached at multiple levels — browser, OS, router, ISP — flush all when debugging
TTL determines cache duration — lower before migrations, raise after for performance
A record for IPv4, AAAA for IPv6, CNAME for aliases, MX for mail
CNAME cannot exist at zone apex (root domain) — use A record or provider-specific alias
dig and nslookup query DNS directly — bypass local cache for accurate results