Address Resolution (DNS and ARP)

Context: FIT1047_MOC · translating BETWEEN the layer addresses — name→IP (DNS, application layer) and IP→MAC (ARP, data link layer) · addresses themselves in Network Addresses (URL, Port, IP, MAC)

Quick Revision

• 🎯 Objective: browsing www.google.com.au needs TWO resolutions ➔ DNS turns the name into $\mathrm{216.58.220.99}$; at the destination LAN, ARP turns that IP into the MAC for frame delivery.
• ⚡ Key Constraint: iterative vs recursive DNS — WHO does the walking: the client (iterative, servers answer “ask X next”) or the DNS server (recursive, often with ISP caching).

📝 Core

DNS (Domain Name System) — name → IP

• What ➔ application-layer protocol over a distributed database of names; servers arranged in a hierarchy mirroring domain structure: root zone → TLD (.au, .com, .edu) → organisation (unsw, unimelb) → host.
• Iterative lookup ➔ client asks root → “ask .au” → client asks TLD → “ask edns-396.unimelb.edu.au” → client asks that server → gets $\mathrm{124.47.170.46}$ — the CLIENT follows each referral.
• Recursive lookup ➔ client asks one server (typically the ISP’s) which does the whole walk itself and returns the final answer — enabling the ISP cache that answers repeat queries instantly.

ARP (Address Resolution Protocol) — IP → MAC

• The last-hop problem ➔ the destination LAN’s router holds a packet for $\mathrm{1.2.3.5}$ but frames need a MAC.
• Mechanism ➔ broadcast “Who has $\mathrm{1.2.3.5}$?” to the whole LAN; the owner replies with its MAC (00:23:ae:e7:52:85); sender frames and delivers.

⚖️ Core Decision Matrix

	DNS	ARP
maps	domain name → IP	IP → MAC
layer	Application	Data Link
scope	global, hierarchical servers	one LAN, broadcast
caching	ISP/OS caches answers	per-host ARP cache

⚠️ Common Mistakes

• 💡 Two resolutions per page view ➔ DNS once (name→IP), ARP at every LAN hop’s end (IP→MAC) — answers claiming one mechanism does both lose the mark.
• 💡 ARP is broadcast by necessity ➔ you can’t unicast to a MAC you don’t yet know; the question IS the discovery.
• 💡 Iterative ≠ recursive by answer quality ➔ both end with the same IP; they differ in who performs the chain (client vs server).

🧠 Active Recall

Trace every resolution step from typing www.unimelb.edu.au to the first frame arriving at the web server.

Answer

• Short answer: DNS (iterative: root → .au TLD → unimelb’s server, or recursive via ISP cache) yields $\mathrm{124.47.170.46}$ → packet routes there → destination router ARP-broadcasts “who has $\mathrm{124.47.170.46}$?” → server replies with its MAC → frame delivered.
• Why: Each layer resolves one hop of identity ➔ names are for humans, IPs for global routing, MACs for local delivery.

Why does the recursive model dominate consumer internet?

Answer

• Short answer: The ISP’s resolver does the walking once and caches; thousands of customers reuse the answer with zero upstream queries.
• Why: Shared cache economics ➔ popular names hit cache almost always, cutting latency and root/TLD load.