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ARP Protocol: What It Maps and Why

ARP Protocol: What It Maps and Why

Verified Sources
Nov 3, 2026

Address Resolution Protocol (ARP) is used to discover the Media Access Control (MAC) address associated with an Internet Protocol version 4 (IPv4) address on the same local link (LAN/broadcast domain). ARP works by broadcasting an ARP request for a target IPv4 address and having the matching host respond with its MAC address, enabling the sender to transmit Ethernet frames correctly.

In short, ARP directly supports the mapping (i) Mapping MAC to IP only in a practical cache sense, while the protocol’s core function is (iii) Mapping IP to MAC. It does not provide mapping (ii) port to process or (iv) host to domain; those are handled by other layers/services (transport/process management and DNS).

Mermaid overview:

Footnotes

  1. Address Resolution Protocol - Wikipedia - Defines ARP as resolving the link-layer (MAC) address associated with an IPv4 address using broadcast request and reply behavior. ↩ ↩2

ARP Explained - Address Resolution Protocol

Correct option(s): (i) and (iii) — with important nuance

Why (iii) is correct: ARP provides IP → MAC mapping

ARP is explicitly defined for discovering the link-layer address (e.g., MAC) associated with an internet-layer address (e.g., IPv4).

The mechanism is:

  • Sender does not know the destination host’s MAC address.
  • Sender broadcasts an ARP request for the destination IPv4 address.
  • The device that owns that IPv4 address replies with its MAC address.
  • The sender records the resulting ARP cache / translation table entry and transmits link-layer frames.

Key keywords: ARP cache; ARP request; ARP reply; local link

Why (i) is “directionally” close, but not ARP’s primary purpose

ARP itself is fundamentally an IP → MAC resolution protocol, but devices typically store the result in an ARP cache that you can sometimes query “the other way” (MAC-to-IP lookups) only because the cache contains both. This makes (i) “Mapping MAC to IP” plausible as a system behavior, but not as ARP performing a formal MAC→IP translation service. The authoritative ARP definition focuses on discovering MAC from IP.

So:

  • (iii) is correct in the protocol definition sense.
  • (i) can appear correct in practice due to cached mappings, but ARP does not natively “resolve MAC→IP” as its standard function.

Key keywords: translation table; cache lookup; cache entry

Footnotes

  1. Address Resolution Protocol - Wikipedia - Defines ARP as resolving the link-layer (MAC) address associated with an IPv4 address using broadcast request and reply behavior. ↩ ↩2 ↩3 ↩4

Why (ii) and (iv) are incorrect: what ARP is not responsible for

(ii) “Mapping port to process” — handled by transport + OS networking

A “port” (TCP/UDP port number) identifies an endpoint in transport-layer protocols, and the OS maps ports to application processes via the host’s networking stack and socket table. ARP operates at link-layer address resolution (MAC discovery), not at transport-layer demultiplexing. ARP’s stated purpose is IP-to-MAC address discovery.

Key keywords: socket; transport layer; demultiplexing

(iv) “Mapping host to domain” — handled by DNS, not ARP

Mapping a host name (or domain) to an IP address is the job of Domain Name System (DNS) and related name-resolution mechanisms. ARP does not translate domains to IPs; it only maps IPv4 addresses to MAC addresses on a local link. ARP’s core description is about link-layer discovery for IPv4 delivery.

Key keywords: DNS; name resolution; hostname

Footnotes

  1. Address Resolution Protocol - Wikipedia - Defines ARP as resolving the link-layer (MAC) address associated with an IPv4 address using broadcast request and reply behavior. ↩ ↩2

Which statement ARP supports (and which it doesn’t)

Scored by direct alignment with ARP’s defined function: link-layer address discovery for IPv4.

ARP IP → MAC resolution (IPv4, same local link)

  1. 1
    Step 1

    If the destination IPv4 already has an entry, use the stored MAC and send the Ethernet frame.

  2. 2
    Step 2

    If there’s no mapping, broadcast an ARP request asking for the MAC address for the target IPv4.

  3. 3
    Step 3

    The host that owns the target IPv4 responds with an ARP reply containing its MAC address.

  4. 4
    Step 4

    The requester stores the learned mapping in its ARP cache/translation table.

  5. 5
    Step 5

    "Now the sender can encapsulate the packet into a frame addressed to the resolved MAC." (ARP request broadcast + MAC reply behavior is described in standard explanations of ARP operation.)

    Footnotes

    1. Address Resolution Protocol - Wikipedia - Defines ARP as resolving the link-layer (MAC) address associated with an IPv4 address using broadcast request and reply behavior. ↩

Pro Tip

"When diagnosing “can’t reach a LAN host,” verify whether the sender’s ARP cache has a reachable entry; an ARP cache miss triggers broadcast ARP to learn the MAC. ARP is specifically about link-layer delivery readiness for IPv4 on the local network."

Footnotes

  1. Address Resolution Protocol - Wikipedia - Defines ARP as resolving the link-layer (MAC) address associated with an IPv4 address using broadcast request and reply behavior. ↩

Common confusion

"ARP is not a name resolver (DNS) and not a transport demultiplexer (ports). If you’re troubleshooting a wrong application/port or a wrong domain name, ARP is usually not the root cause."

Footnotes

  1. Address Resolution Protocol - Wikipedia - Defines ARP as resolving the link-layer (MAC) address associated with an IPv4 address using broadcast request and reply behavior. ↩

Answering the multiple-choice prompt directly

Knowledge Check

Question 1 of 3
Q1Single choice

Which mapping does ARP directly provide on IPv4 networks?

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