This 200-901 practice question tests your understanding of network fundamentals. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
An engineer is troubleshooting packet loss between two hosts on different subnets. The traceroute shows that packets reach the first hop router but then stop. The router's ARP table shows an incomplete entry for the next-hop IP address. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The next-hop device is powered off or has a Layer 1 issue.
An incomplete ARP entry means the router sent an ARP request but received no reply, indicating the next-hop device is unreachable at Layer 2. Option A is wrong because ACL would drop packets differently. Option B is wrong because MTU issues cause fragmentation or drops, not ARP failure. Option C is wrong because routing protocol convergence would not cause ARP failure specifically.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The MTU is misconfigured on the outgoing interface.
Why it's wrong here
MTU issues cause packet drops but not ARP incompleteness.
✗
The routing protocol has not converged yet.
Why it's wrong here
Routing convergence affects the route table, not ARP.
✓
The next-hop device is powered off or has a Layer 1 issue.
Why this is correct
If the next-hop device is offline, the router cannot complete ARP.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
An ACL is blocking traffic on the outgoing interface.
Why it's wrong here
ACLs drop packets after routing, they don't prevent ARP resolution.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 200-901 subnetting questions on CIDR, address ranges, and subnet selection.
Network Fundamentals — This question tests Network Fundamentals — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The next-hop device is powered off or has a Layer 1 issue. — An incomplete ARP entry means the router sent an ARP request but received no reply, indicating the next-hop device is unreachable at Layer 2. Option A is wrong because ACL would drop packets differently. Option B is wrong because MTU issues cause fragmentation or drops, not ARP failure. Option C is wrong because routing protocol convergence would not cause ARP failure specifically.
What should I do if I get this 200-901 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 200-901 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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