The answer is the guest OS firewall on VM2 blocking inbound port 8080. This is correct because Azure Network Security Groups (NSGs) operate at the network layer, filtering traffic before it reaches the VM’s operating system, while the guest OS firewall—such as Windows Firewall—runs inside the VM and controls application-layer access independently. Even when an NSG rule explicitly allows port 8080, the guest OS firewall can still drop the traffic, which explains why ICMP ping succeeds (it uses a different protocol) but the application connection fails. On the AZ-500 exam, this scenario tests your understanding of defense-in-depth and the layered security model, where NSGs and OS firewalls serve distinct roles. A common trap is assuming an NSG allow rule guarantees connectivity, but the guest OS firewall is a separate control that must also permit the traffic. Memory tip: think of the NSG as the bouncer at the building entrance, and the guest OS firewall as the bouncer at the specific apartment door—both must say yes for the application to get through.
AZ-500 Secure networking Practice Question
This AZ-500 practice question tests your understanding of secure networking. 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.
You are troubleshooting connectivity between two Azure VMs in the same virtual network. VM1 can ping VM2, but VM1's application cannot connect to VM2's application on port 8080. Both VMs have NSGs that allow inbound traffic on port 8080. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The guest OS firewall on VM2 is blocking inbound port 8080.
Option B is correct because Windows Firewall runs inside the OS and can block application ports even if Azure NSGs allow traffic. Option A is wrong because ICMP (ping) works. Option C is wrong because NSGs allow the port. Option D is wrong because a load balancer would not affect direct traffic.
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 VNet is peered with another VNet that has a conflicting address space.
Why it's wrong here
Peering conflicts would not affect intra-VNet traffic.
✗
An Azure Load Balancer is directing traffic away from VM2.
Why it's wrong here
Load balancers do not affect direct VM-to-VM traffic.
✗
The NSG on VM2's subnet has a deny rule for port 8080.
Why it's wrong here
The NSG allows port 8080 per the scenario.
✓
The guest OS firewall on VM2 is blocking inbound port 8080.
Why this is correct
Guest OS firewall can block traffic even if NSGs allow.
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
Scenario analysis trap
The NSG allows port 8080 per the scenario.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this AZ-500 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 AZ-500 subnetting questions on CIDR, address ranges, and subnet selection.
Secure networking — This question tests Secure networking — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The guest OS firewall on VM2 is blocking inbound port 8080. — Option B is correct because Windows Firewall runs inside the OS and can block application ports even if Azure NSGs allow traffic. Option A is wrong because ICMP (ping) works. Option C is wrong because NSGs allow the port. Option D is wrong because a load balancer would not affect direct traffic.
What should I do if I get this AZ-500 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 AZ-500 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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