ANS-C01 Network Implementation Practice Question
A company has a VPC with a CIDR of 10.0.0.0/16 and has enabled VPC Flow Logs to capture all traffic. The logs show that an EC2 instance (10.0.1.10) is sending outbound traffic to an external IP (203.0.113.50) on port 443, but the traffic is being rejected. The instance's security group allows outbound HTTPS to 0.0.0.0/0, and the subnet's NACL allows outbound traffic on port 443. The VPC has an internet gateway attached, and the route table directs 0.0.0.0/0 to the internet gateway. What is the most likely cause of the rejection?
⚠ Common exam trap
AWS often tests the distinction between stateful security groups and stateless NACLs, specifically that NACLs require explicit inbound rules for return traffic while security groups automatically allow it.
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 NACL inbound rules are blocking the return traffic.
The outbound traffic from the EC2 instance is allowed by the security group and NACL outbound rules, but the return traffic (responses from 203.0.113.50:443 back to 10.0.1.10) must also be permitted by the subnet's NACL inbound rules. Since the NACL is stateless, it requires explicit inbound rules for ephemeral ports (typically 1024-65535) to allow the return traffic. If those inbound rules are missing, the return packets are dropped, causing the outbound traffic to appear as rejected in VPC Flow Logs.
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 NACL inbound rules are blocking the return traffic.
Why this is correct
NACLs are stateless and require explicit inbound rules for return traffic.
- ✗
The security group does not allow inbound HTTPS traffic.
Why it's wrong here
Security groups are stateful; outbound allowance implies inbound return traffic is allowed.
- ✗
The internet gateway is not attached to the VPC.
Why it's wrong here
It is attached.
- ✗
The route table does not have a route to the internet gateway.
Why it's wrong here
Route is present.
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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.