The answer is the security group attached to the instance does not allow traffic from the load balancer. Even though the load balancer is internet-facing and resides in the same VPC and subnet, it communicates with backend instances using their private IP addresses, so the instance does not need a public IP. However, the instance’s security group must explicitly permit inbound HTTP or HTTPS traffic from the load balancer’s security group or the VPC CIDR; without that rule, the load balancer’s health checks and forwarded requests are silently dropped. On the CompTIA Cloud+ CV0-004 exam, this scenario tests your understanding of how security groups act as a virtual firewall at the instance level, independent of subnet routing or public IP assignment. A common trap is assuming a private subnet blocks all traffic, but the real blocker is often a missing inbound rule. Memory tip: “Private IP, public rule” — the instance stays private, but the security group must open the door for the load balancer.
CV0-004 Troubleshooting Practice Question
This CV0-004 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.
Network Topology
A cloud administrator is troubleshooting why web-server-01 is not receiving traffic from an internet-facing load balancer. The load balancer is in the same VPC and subnet. According to the exhibit, what is the most likely reason?
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 security group attached to the instance does not allow traffic from the load balancer
The instance has no public IP address (not shown) and the security group 'web-sg' may not allow HTTP traffic. However, the exhibit does not show security group rules or public IP. The most likely issue is that the instance is in a private subnet without a route to the internet? But the question says load balancer is internet-facing and in same VPC. The load balancer can route to private instances if security groups allow. However, the exhibit shows no public IP, so the load balancer can still reach it via private IP. Actually, the most common issue is security group rules: the load balancer's security group must allow inbound HTTP, and the instance's security group must allow traffic from the load balancer. Without that, traffic is blocked. Option C is correct. Option A is wrong because the load balancer can route to private IPs. Option B is wrong because the instance is running. Option D is wrong because load balancer does not require a public IP on the instance.
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 security group attached to the instance does not allow traffic from the load balancer
Why this is correct
The security group must allow inbound HTTP/HTTPS from the load balancer's security group or CIDR.
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
The instance is in the same VPC as shown.
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.
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 CV0-004 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 security group attached to the instance does not allow traffic from the load balancer — The instance has no public IP address (not shown) and the security group 'web-sg' may not allow HTTP traffic. However, the exhibit does not show security group rules or public IP. The most likely issue is that the instance is in a private subnet without a route to the internet? But the question says load balancer is internet-facing and in same VPC. The load balancer can route to private instances if security groups allow. However, the exhibit shows no public IP, so the load balancer can still reach it via private IP. Actually, the most common issue is security group rules: the load balancer's security group must allow inbound HTTP, and the instance's security group must allow traffic from the load balancer. Without that, traffic is blocked. Option C is correct. Option A is wrong because the load balancer can route to private IPs. Option B is wrong because the instance is running. Option D is wrong because load balancer does not require a public IP on the instance.
What should I do if I get this CV0-004 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 CV0-004 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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