The correct answer is an ARP request with the target IP set to its own IP address. This packet, known as a gratuitous ARP request, is the foundation of Duplicate Address Detection (DAD) because it forces any host already using that IP to reply, instantly revealing a conflict. On the JNCIA-Junos exam, this concept tests your understanding of how Junos handles IPv4 address assignment and conflict resolution—a common trap is confusing gratuitous ARP with a standard ARP request or reply. Remember, the key distinction is that the sender’s target IP matches its own source IP, making it a self-addressed probe rather than a lookup for another device. For a quick memory tip: think “Gratuitous ARP = Giving your own IP as the target to check for a ghost.”
JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking fundamentals. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
ARP reply with its own MAC address
Why it's wrong here
An ARP reply is only sent in response to an ARP request.
✓
ARP request with target IP set to its own IP
Why this is correct
This is the standard method for duplicate address detection, where the host sends an ARP request for its own IP.
Related concept
Read the scenario before looking for a memorised answer.
✗
Gratuitous ARP reply
Why it's wrong here
A gratuitous ARP reply is sent to update caches, not for duplicate detection.
✗
ARP request with target IP set to the default gateway
Why it's wrong here
This is used to resolve the gateway's MAC address, not for duplicate detection.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often 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.
Detailed technical explanation
How to think about this question
Under the hood, the host sends an ARP request with the sender IP set to 0.0.0.0 (or its tentative IP) and the target IP set to its own IP. If no ARP reply is received within a timeout period (typically 1 second), the address is considered unique. This behavior is defined in RFC 5227 for IPv4 Address Conflict Detection (ACD), and on Junos, the 'duplicate-address-detection' feature can be configured to handle such conflicts automatically.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the JNCIA-JUNOS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking Fundamentals — This question tests Networking Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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