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.
A company has a DHCP server that assigns IP addresses from a scope of 192.168.10.0/24. A new device receives IP 192.168.10.100/24 but cannot access the internet. The default gateway is 192.168.10.1. What is the most likely issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
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 default gateway is not reachable from the device.
The device received a valid IP address (192.168.10.100/24) and subnet mask from the DHCP server, but it cannot access the internet. Since the default gateway is 192.168.10.1, the most likely cause is that the device cannot reach the gateway, which is required to route traffic outside the local subnet. Without connectivity to the default gateway, the device cannot forward packets to external networks, even though its IP configuration is otherwise correct.
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.
✗
DNS server is unreachable.
Why it's wrong here
Even with DNS issues, IP-based access would work if gateway is reachable.
✗
The DHCP scope is exhausted.
Why it's wrong here
The device received an IP, so scope is not exhausted.
✗
The device has a duplicate IP address.
Why it's wrong here
Duplicate IP would cause intermittent issues, not complete inability to access internet.
✓
The default gateway is not reachable from the device.
Why this is correct
This is the most direct cause: if the gateway is down or not on the same VLAN, traffic cannot exit.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The device's subnet mask is incorrect.
Why it's wrong here
/24 is correct for that range.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept that a valid IP address and subnet mask do not guarantee internet access; the default gateway must be reachable, and candidates may mistakenly blame DNS or DHCP exhaustion when the real issue is Layer 3 connectivity to the gateway.
Detailed technical explanation
How to think about this question
When a device cannot reach its default gateway, it may be due to a missing or incorrect ARP entry for the gateway IP, a misconfigured switch port (e.g., VLAN mismatch or port security), or a firewall rule blocking ICMP or routed traffic. In real-world scenarios, this often occurs when the gateway's MAC address is not resolved via ARP, or when the gateway itself is down or has a routing table issue. The device's ability to ping other hosts on the same subnet (e.g., 192.168.10.2) would succeed, but any traffic destined for a different subnet would fail because the gateway is unreachable at Layer 2.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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.
Network Fundamentals — This question tests Network Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The default gateway is not reachable from the device. — The device received a valid IP address (192.168.10.100/24) and subnet mask from the DHCP server, but it cannot access the internet. Since the default gateway is 192.168.10.1, the most likely cause is that the device cannot reach the gateway, which is required to route traffic outside the local subnet. Without connectivity to the default gateway, the device cannot forward packets to external networks, even though its IP configuration is otherwise correct.
What should I do if I get this 200-901 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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