JN0-106 Networking Fundamentals Practice Question
A host needs to verify that its assigned IP address is not already in use on the network. Which type of packet does the host send?
⚠ Common exam trap
It's easy for candidates to confuse a gratuitous ARP reply (used to announce an address) with a gratuitous ARP request (used for duplicate address detection), leading them to select option C instead of B.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
ARP request with target IP set to its own IP
When a host wants to verify that its assigned IP address is not already in use on the network, it sends an ARP request with the target IP set to its own IP address. This is known as a gratuitous ARP request, and if another host responds with an ARP reply, it indicates an IP address conflict. This process is part of the Duplicate Address Detection (DAD) mechanism, commonly used in IPv4 networks to ensure uniqueness before the address is fully configured.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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ARP reply with its own MAC address
Why it's wrong here
ARP replies are always responses to a prior ARP request; they are never used to initiate a probe. For duplicate address detection, the host must actively query the network with an ARP request, not passively receive a reply. An ARP reply carrying the host's own MAC address would only occur if the host had already sent a request, and it would indicate a conflicting responder, not the detection mechanism itself.
- ✓
ARP request with target IP set to its own IP
Why this is correct
The standard DAD probe is an ARP request with the sender IP and protocol address set to 0.0.0.0 and the target IP set to the host's own intended address, broadcast to the local segment. Per RFC 5227, the host then waits to see whether any device answers; if a reply or another ARP announcement arrives, the address is already in use. This is often called an ARP probe, and it explicitly asks the network 'does anyone own this IP?' before the host commits to it.
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Gratuitous ARP reply
Why it's wrong here
A gratuitous ARP reply is an unsolicited broadcast announcement that maps the sender's own IP address to its MAC address, typically sent to refresh neighboring hosts' ARP caches or inform them of a link-layer address change. Unlike the DAD probe, it does not query for a conflict; it simply asserts an address ownership, which would actually create a conflict if another host already held that IP. It is not designed to elicit responses, so the originating host cannot use it to detect a duplicate.
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ARP request with target IP set to the default gateway
Why it's wrong here
When a host needs to send a packet to a different subnet, it must deliver the frame to the default gateway's MAC address, so it sends an ARP request whose target IP is the gateway's address. Such a request is a standard address-resolution query and has no bearing on duplicate address detection for the host's own IP. The response will supply the gateway's MAC, and a conflict with the host's own address would never be revealed by this exchange.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This JN0-106 practice question is part of Courseiva's free Juniper Networks certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the JN0-106 exam.