GATE/Computer Networks/Application Layer — DNS, HTTP & Email
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Easy15 min readComputer Networks

Application Layer — DNS, HTTP & Email

The application layer provides user-facing network services. GATE tests DNS resolution, HTTP methods and headers, email protocols (SMTP/POP3/IMAP), and DHCP.

Key Points

  • ·DNS: hierarchical naming system; root → TLD → authoritative → local resolver
  • ·DNS resolution: recursive (resolver queries on behalf of client) vs iterative (client queries each server)
  • ·DNS record types: A (IPv4), AAAA (IPv6), MX (mail), CNAME (alias), NS (name server), PTR (reverse)
  • ·HTTP: stateless, request-response over TCP (port 80); HTTPS = HTTP + TLS (port 443)
  • ·HTTP methods: GET (retrieve), POST (submit), PUT (replace), DELETE, HEAD, OPTIONS
  • ·HTTP 1.1: persistent connections, pipelining; HTTP/2: multiplexing, header compression
  • ·SMTP: sending email (port 25/587); POP3: download email (port 110); IMAP: sync email (port 143)
  • ·DHCP: automatically assigns IP, subnet mask, gateway, DNS server — uses UDP ports 67/68
  • ·Cookies: HTTP state management (stateless protocol workaround)

Key Formulas

  • DNS TTL: Cache valid for TTL seconds; stale after TTL expires
  • DHCP DORA: Discover → Offer → Request → Acknowledge

GATE Exam Tips

  • ★DNS uses UDP for queries (fast, small); TCP for zone transfers (reliable, large).
  • ★HTTP is stateless — cookies and sessions are application-layer workarounds.
  • ★POP3 deletes from server after download; IMAP keeps on server — exam loves this distinction.
  • ★DHCP uses broadcast (255.255.255.255) because client doesn't have IP yet — UDP based.

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