3Network applications

• 3.1DNS

  Covers the division of roles between authoritative servers and caching (resolving) servers, how zones and delegation partition the domain namespace, resource records such as A/AAAA/MX/CNAME/NS/PTR, the difference between recursive queries and iterative queries, and how to triage DNS failures using forward and reverse lookups.

• 3.2DHCP and address distribution

  Covers the DHCP four-message exchange (DISCOVER/OFFER/REQUEST/ACK), temporary address assignment via lease, the DHCP relay agent used across multiple segments, static (reserved) allocation for servers and similar devices, and DHCP scope design decisions in a multi-segment environment.

• 3.3HTTP, web, and mail protocols

  Covers the fundamentals of HTTP/HTTPS, the performance improvements of HTTP/2 and HTTP/3 (QUIC), cookie/session management, mail transfer via SMTP/POP3/IMAP, sender domain authentication via SPF/DKIM/DMARC, and mail attachments via MIME, together with design decisions for web/mail performance and anti-spoofing measures.

• 3.4Load balancing and content delivery

  Covers the difference between L4 and L7 load balancers, distribution methods such as round robin, least connections, and weighted algorithms, automatic removal of failed servers via health checks, session persistence (sticky sessions), content delivery optimization via reverse proxies and CDNs, and time synchronization via NTP, together with the judgment of choosing L4 vs. L7 for a given requirement.