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ARP (Address Resolution Protocol): Mapping IP Addresses to MAC Addresses

ARP (Address Resolution Protocol): Mapping IP Addresses to MAC Addresses

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Nov 4, 2026

When a host wants to send an IPv4 packet to an IP address on the same local network, it must know the destination’s link-layer (MAC) address. Because IPv4 addresses don’t directly identify MAC addresses on Ethernet, the host uses Address Resolution Protocol (ARP) to build or consult an ARP cache.

At a high level:

  • The sender checks its ARP cache for the target IP.
  • If missing, it sends an ARP request to the local network.
  • The target host responds with an ARP reply.
  • The sender updates its ARP cache and then transmits the actual data frame to the discovered MAC address.

A canonical specification for ARP is provided in RFC 826 . ARP is carried within Ethernet frames; ARP requests are typically sent with Ethernet broadcast so that all nodes on the local network receive the query .

[CalloutBlock type="tip" title="Pro Tip"]
ARP is local-network (same broadcast domain). If the destination is remote (different network), the sender typically resolves the default gateway’s MAC address, not the final host’s MAC. This behavior follows from how IP routing chooses the next hop.
[/CalloutBlock]

Footnotes

  1. [RFC 826: An Ethernet Address Resolution Protocol] (https://www.rfc-editor.org/rfc/rfc826) - Defines ARP packet structure and request/reply semantics. ↩

  2. [Ethernet broadcast address (FF:FF:FF:FF:FF:FF) and broadcast behavior] (https://en.wikipedia.org/wiki/MAC_address#Broadcast_address) - Explains the broadcast MAC address used to deliver frames to all hosts on the LAN. ↩

  3. [IPv4 routing and next hop concept (overview)] (https://en.wikipedia.org/wiki/Routing#Next_hop) - Explains that routing selects a next hop, after which link-layer resolution targets the next hop on the local network. ↩

Key ARP concepts and terms (quick reference)

  • ARP cache: Stores recent mappings to avoid repeated ARP.
  • ARP request: Sent when mapping is unknown.
  • ARP reply: Returned by the host that owns the queried IP.
  • Broadcast domain: ARP requests are broadcast within this domain.
  • Ethernet destination MAC: For ARP requests, it is typically the broadcast MAC.

These terms appear naturally in the ARP request/reply behavior defined in RFC 826 and the Ethernet broadcast mechanism used to reach all nodes .

Footnotes

  1. [RFC 826: An Ethernet Address Resolution Protocol] (https://www.rfc-editor.org/rfc/rfc826) - Defines ARP packet structure and request/reply semantics. ↩

  2. [Ethernet broadcast address (FF:FF:FF:FF:FF:FF) and broadcast behavior] (https://en.wikipedia.org/wiki/MAC_address#Broadcast_address) - Explains the broadcast MAC address used to deliver frames to all hosts on the LAN. ↩

ARP request and reply workflow (IP-to-MAC resolution)

  1. 1
    Step 1

    Host A looks for the Target IP in its ARP cache. If present, it immediately uses the cached MAC for the next Ethernet frame.

  2. 2
    Step 2

    If the entry is missing, Host A constructs an ARP request for the Target IP.

  3. 3
    Step 3

    Host A sends the ARP request inside an Ethernet frame whose destination MAC is broadcast (commonly FF:FF:FF:FF:FF:FF), so all hosts on the local LAN can receive it .

    Footnotes

    1. [Ethernet broadcast address (FF:FF:FF:FF:FF:FF) and broadcast behavior] (https://en.wikipedia.org/wiki/MAC_address#Broadcast_address) - Explains the broadcast MAC address used to deliver frames to all hosts on the LAN. ↩

  4. 4
    Step 4

    Each host on the LAN examines the ARP request. The host whose IPv4 address matches the requested Target IP recognizes the query.

  5. 5
    Step 5

    The matching host responds with an ARP reply to Host A, typically using Host A’s MAC address carried in the request.

  6. 6
    Step 6

    Host A stores the received mapping (Target IP → Target MAC) in its ARP cache for future transmissions.

  7. 7
    Step 7

    Finally, Host A sends the actual IP packet encapsulated in an Ethernet frame addressed to the resolved Target MAC.

What’s inside the ARP request and reply (conceptual fields)

ARP messages include fields that bind together:

  • the hardware type and protocol type (e.g., Ethernet + IPv4),
  • the sender hardware address (MAC) and sender protocol address (IP),
  • the target hardware address and target protocol address (the IP being resolved).

RFC 826 defines ARP packet structure and semantics . The important operational detail for mapping is:

  • In an ARP request, the target MAC is generally unknown (set to a placeholder) because the sender is still trying to resolve it .
  • In an ARP reply, the target MAC is filled in with the real MAC address corresponding to the target IP .

[CalloutBlock type="warning" title="Why broadcast is required"]

If Host A doesn’t already know the target’s MAC, there is no Layer 2 address to send a unicast frame to. So ARP uses broadcast to ensure the correct host can hear the question. This broadcast behavior is the mechanism that makes ARP work in a local Ethernet broadcast domain .
[/CalloutBlock]

Footnotes

  1. [RFC 826: An Ethernet Address Resolution Protocol] (https://www.rfc-editor.org/rfc/rfc826) - Defines ARP packet structure and request/reply semantics. ↩ ↩2 ↩3

  2. [Ethernet broadcast address (FF:FF:FF:FF:FF:FF) and broadcast behavior] (https://en.wikipedia.org/wiki/MAC_address#Broadcast_address) - Explains the broadcast MAC address used to deliver frames to all hosts on the LAN. ↩

ARP in time: from cache miss to data delivery

Cache lookup

T0

Sender checks ARP cache for Target IP."

Cache miss → ARP request

T1

Sender broadcasts ARP request: “Who has Target IP?”"

Target host responds

T2

Target unicasts ARP reply with IP→MAC mapping."

Cache update

T3

Sender stores the mapping in ARP cache."

Original IP packet sent

T4

Sender sends the IP packet in an Ethernet frame to resolved MAC."

AR P exchange roles in the workflow

How each participant contributes during resolution (conceptual).

Common exam/implementation details

ARP Explained (Address Resolution Protocol) — Request/Reply

Knowledge Check

Question 1 of 3
Q1Single choice

When Host A does not know the MAC address for a target IPv4 address on the same local network, what does it send?

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