The answer is the security group only allows traffic from the 10.0.0.0/8 range. This is the most likely cause because security groups act as a virtual firewall at the instance level, and the inbound rule permitting HTTP traffic is restricted to a private IP range, which blocks any requests originating from the public internet. Since the EC2 instance resides in a public subnet with an Internet Gateway and a default route to it, the routing and network ACLs are correctly configured, leaving the security group’s source restriction as the sole barrier. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your ability to differentiate between security group rules and network ACLs, a common trap where candidates overlook that security groups are stateful and must explicitly allow inbound traffic from 0.0.0.0/0 for internet access. A quick memory tip: if your instance can’t be reached from the internet but local traffic works, always check the security group’s source—private ranges like 10.0.0.0/8 are for internal use only.
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 network engineer is troubleshooting why an EC2 instance (with the above security group) is not responding to HTTP requests from the internet. The instance is in a public subnet with an Internet Gateway attached. The route table has a default route to the Internet Gateway. 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 security group only allows traffic from the 10.0.0.0/8 range
The security group allows HTTP from 10.0.0.0/8 only, which is a private IP range. It does not allow traffic from 0.0.0.0/0 (the internet). Option B is correct. Options A, C, and D are not the issue.
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 only allows traffic from the 10.0.0.0/8 range
Why this is correct
The security group rule only permits HTTP from the private 10.0.0.0/8 CIDR, not from the internet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The route table does not have a route for the internet
Why it's wrong here
The route table has a default route to the Internet Gateway per the stem.
✗
The network ACL is blocking inbound HTTP
Why it's wrong here
No information about NACLs, and security groups are the first line of defense.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group 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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group only allows traffic from the 10.0.0.0/8 range — The security group allows HTTP from 10.0.0.0/8 only, which is a private IP range. It does not allow traffic from 0.0.0.0/0 (the internet). Option B is correct. Options A, C, and D are not the issue.
What should I do if I get this ANS-C01 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 ANS-C01 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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