This ANS-C01 practice question tests your understanding of network implementation. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 security team wants to block traffic from a specific IP address (203.0.113.5) from reaching an EC2 instance. The instance is in a public subnet with a security group that allows all traffic from the internet. A network ACL is associated with the subnet. The team adds a DENY rule for the IP in the network ACL. However, traffic from that IP still reaches the instance. 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 network ACL has an ALLOW rule with a lower rule number that matches the IP, so the DENY rule is never evaluated.
Option D is correct because network ACLs are stateless and require both inbound and outbound rules to allow return traffic. The DENY rule on inbound will block incoming traffic, but if the ACL also has an inbound ALLOW rule for the IP, the DENY rule might be evaluated after the ALLOW rule if the rule numbers are ordered incorrectly. However, the question states they added a DENY rule; if the ALLOW rule has a lower rule number, it will be evaluated first. Option A is wrong because security groups are stateful, but they do not override network ACLs. Option B is wrong because network ACLs apply to the subnet, not the instance. Option C is wrong because the internet gateway operates at layer 3 and does not filter by IP.
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 network ACL has an ALLOW rule with a lower rule number that matches the IP, so the DENY rule is never evaluated.
Why this is correct
Network ACLs are processed in rule number order; the first matching rule is applied.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group allows traffic from the IP, overriding the network ACL.
Why it's wrong here
Security groups and network ACLs work together; security group allows, but network ACL blocks if DENY is evaluated first.
✗
The network ACL is applied to the wrong subnet.
Why it's wrong here
If it's applied to the subnet, it affects all instances in that subnet.
✗
The internet gateway is ignoring the network ACL.
Why it's wrong here
Internet gateway does not filter; it forwards packets based on route tables.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The network ACL has an ALLOW rule with a lower rule number that matches the IP, so the DENY rule is never evaluated. — Option D is correct because network ACLs are stateless and require both inbound and outbound rules to allow return traffic. The DENY rule on inbound will block incoming traffic, but if the ACL also has an inbound ALLOW rule for the IP, the DENY rule might be evaluated after the ALLOW rule if the rule numbers are ordered incorrectly. However, the question states they added a DENY rule; if the ALLOW rule has a lower rule number, it will be evaluated first. Option A is wrong because security groups are stateful, but they do not override network ACLs. Option B is wrong because network ACLs apply to the subnet, not the instance. Option C is wrong because the internet gateway operates at layer 3 and does not filter by IP.
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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