ANS-C01 Network Design Practice Question
A company has a VPC peering connection between VPC A (10.0.0.0/16) and VPC B (10.1.0.0/16). Both VPCs have subnets with EC2 instances. The security groups allow all traffic between the instances. The instances in VPC A can ping the instances in VPC B, but cannot initiate TCP connections to a web server running on port 443 in VPC B. What is the most likely cause?
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 security group for the web server in VPC B does not allow inbound traffic from VPC A on TCP port 443.
The security group for the web server in VPC B must explicitly allow inbound traffic from VPC A on TCP port 443. Even though ICMP (ping) works because security groups by default allow all outbound traffic and ICMP may be permitted, TCP connections require an explicit inbound rule. Option A is incorrect because VPC peering supports TCP traffic. Option B is incorrect because DNS resolution settings are not required for direct TCP connections over VPC peering. Option D is incorrect because network ACLs are stateless and would block both ICMP and TCP if misconfigured; since ping works, a NACL issue is unlikely.
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 VPC peering connection does not support TCP traffic.
Why it's wrong here
VPC peering supports all protocols.
- ✗
The DNS resolution settings for the VPC peering are not enabled.
Why it's wrong here
DNS resolution is for hostname resolution, not TCP connectivity.
- ✓
The security group for the web server in VPC B does not allow inbound traffic from VPC A on TCP port 443.
Why this is correct
Security groups are stateful; if inbound rule missing, TCP connections are denied while ICMP might be allowed.
- ✗
The network ACL in VPC B is blocking inbound TCP traffic on port 443.
Why it's wrong here
If NACL blocked TCP, ICMP might also be affected if not explicitly allowed; but ICMP works, so unlikely.
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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.