ANS-C01 Network Implementation Practice Question
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?
⚠ Common exam trap
It's easy for candidates to 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.
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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.