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Network Implementation →mediumMultiple ChoiceObjective-mapped

Network ACL DENY Rule Not Blocking Traffic — First Match Wins with Lower ALLOW Rule

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.

A
The network ACL has an ALLOW rule with a lower rule number that matches the IP, so the DENY rule is never evaluated.
Network ACLs are processed in rule number order; the first matching rule is applied.
B
The security group allows traffic from the IP, overriding the network ACL.
Why wrong: Security groups and network ACLs work together; security group allows, but network ACL blocks if DENY is evaluated first.
C
The network ACL is applied to the wrong subnet.
Why wrong: If it's applied to the subnet, it affects all instances in that subnet.
D
The internet gateway is ignoring the network ACL.
Why wrong: Internet gateway does not filter; it forwards packets based on route tables.

Full breakdown with real-world context →

Answer choices

Why each option matters

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.

Network ACLs are stateless and evaluated in order by rule number, from lowest to highest. If an ALLOW rule with a lower rule number (e.g., 100) matches the source IP 203.0.113.5 before the DENY rule (e.g., 200) is reached, the ALLOW rule will be applied and the traffic permitted. The DENY rule is never evaluated because the first matching rule determines the action.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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.
Related concept
Read the scenario before looking for a memorised 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: answer the scenario, not the keyword

The trap here is that candidates assume a DENY rule in a network ACL will always block traffic, forgetting that network ACLs are evaluated in rule-number order and the first matching rule wins, so a lower-numbered ALLOW rule can permit the traffic before the DENY rule is even considered.

Detailed technical explanation

How to think about this question

Network ACLs are evaluated as a stateless packet filter at the subnet boundary, processing rules in ascending order of rule number (e.g., 100, 200). The first rule that matches the packet's source IP, destination IP, protocol, and port determines the action (ALLOW or DENY); subsequent rules are skipped. This is defined in AWS VPC documentation and is analogous to ordered ACLs in traditional networking. A common real-world scenario is when a default ALLOW rule (e.g., rule 100 for all traffic) is present, and a higher-numbered DENY rule (e.g., rule 200) is added but never reached.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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.

Visual reference

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..

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. — Network ACLs are stateless and evaluated in order by rule number, from lowest to highest. If an ALLOW rule with a lower rule number (e.g., 100) matches the source IP 203.0.113.5 before the DENY rule (e.g., 200) is reached, the ALLOW rule will be applied and the traffic permitted. The DENY rule is never evaluated because the first matching rule determines the action.

What should I do if I get this ANS-C01 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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?

Read the scenario before looking for a memorised answer.

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