Local network operation: Which protocol is used to discover the hardware (MAC) address that corresponds to a local IPv4 address on the same LAN?

Introduction / Context:
On Ethernet LANs, devices must map Layer 3 IPv4 addresses to Layer 2 MAC addresses to deliver frames. The protocol that performs this resolution is crucial for basic connectivity and troubleshooting duplicate IPs or stale cache entries.

Given Data / Assumptions:

• We are working with IPv4 on a broadcast LAN (e.g., Ethernet).
• Hosts require a mapping from IP to MAC before sending unicast frames.
• The target IP is in the same subnet as the sender.

Concept / Approach:
Address Resolution Protocol (ARP) broadcasts a query: “Who has IP X? Tell Y.” The owner of IP X responds with its MAC address, allowing Y to populate its ARP table and send frames directly. Reverse ARP (RARP) historically mapped MAC to IP (now superseded by modern boot protocols). ICMP handles control messages, not L2 resolution.

Step-by-Step Solution:

Verification / Alternative check:
Use command line tools (arp -a, ip neigh) to inspect cache entries after communication. Packet captures clearly show ARP Request/Reply exchanges preceding unicast traffic.

Why Other Options Are Wrong:

• RARP maps MAC to IP; largely obsolete.
• IP is the Layer 3 protocol being mapped, not the resolver.
• ICMP is for control messages (e.g., Echo), not address resolution.
• DHCP leases addresses; it does not perform per-packet L2 resolution.

Common Pitfalls:
Assuming ARP works across routers (it does not); ignoring ARP cache timeouts that cause intermittent delays.

Final Answer:
ARP