This XK0-005 practice question tests your understanding of troubleshooting. 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.
Exhibit
User reports: "I can't access the web server at https://example.com from my office network."
From the user's workstation:
$ nslookup example.com
Server: 192.168.1.1
Address: 192.168.1.1#53
Non-authoritative answer:
Name: example.com
Address: 10.0.0.5
From another host on the same subnet:
$ curl -I https://example.com
HTTP/2 200
From the user's workstation:
$ curl -I https://example.com
curl: (7) Failed to connect to example.com port 443: Connection refused
$ iptables -L -n -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
Refer to the exhibit. A user cannot access a web server, but another host on the same subnet can. 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: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
User reports: "I can't access the web server at https://example.com from my office network."
From the user's workstation:
$ nslookup example.com
Server: 192.168.1.1
Address: 192.168.1.1#53
Non-authoritative answer:
Name: example.com
Address: 10.0.0.5
From another host on the same subnet:
$ curl -I https://example.com
HTTP/2 200
From the user's workstation:
$ curl -I https://example.com
curl: (7) Failed to connect to example.com port 443: Connection refused
$ iptables -L -n -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
A
The network router is blocking the user's traffic.
Why wrong: If the router blocked it, the other host on the same subnet would likely also be blocked.
B
The web server is down.
Why wrong: Another host can access it, so the server is up.
C
DNS is resolving to the wrong IP for the user.
Why wrong: nslookup shows the same IP as likely used by the other host, and resolution is consistent.
D
The user's workstation has a local firewall blocking outbound HTTPS.
The iptables output shows no rules, but the user's workstation gets 'Connection refused' while another host succeeds, indicating the issue is local to the workstation. A local firewall (e.g., software firewall) might be blocking outbound 443.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The user's workstation has a local firewall blocking outbound HTTPS.
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.
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.
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 network router is blocking the user's traffic.
Why it's wrong here
If the router blocked it, the other host on the same subnet would likely also be blocked.
✗
The web server is down.
Why it's wrong here
Another host can access it, so the server is up.
✗
DNS is resolving to the wrong IP for the user.
Why it's wrong here
nslookup shows the same IP as likely used by the other host, and resolution is consistent.
✓
The user's workstation has a local firewall blocking outbound HTTPS.
Why this is correct
The iptables output shows no rules, but the user's workstation gets 'Connection refused' while another host succeeds, indicating the issue is local to the workstation. A local firewall (e.g., software firewall) might be blocking outbound 443.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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.
Trap categories for this question
Command / output trap
nslookup shows the same IP as likely used by the other host, and resolution is consistent.
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this XK0-005 question in full detail.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related XK0-005 subnetting questions on CIDR, address ranges, and subnet selection.
Troubleshooting — This question tests Troubleshooting — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The user's workstation has a local firewall blocking outbound HTTPS.
What should I do if I get this XK0-005 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 XK0-005 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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