The correct configuration is a web security group outbound rule to the database security group on port 3306, paired with a database security group inbound rule from the web security group on port 3306. This works because security groups are stateful, meaning that if the web server initiates outbound traffic to the database, the database’s response is automatically allowed back, even without an explicit inbound rule on the web side. However, the database security group must still have an inbound rule explicitly permitting traffic from the web security group, as security groups filter at the instance level and do not automatically accept unsolicited connections. On the AWS Certified Advanced Networking Specialty exam, this scenario tests your understanding of stateful versus stateless filtering and the difference between security groups and network ACLs—a common trap is confusing the two and adding unnecessary inbound rules on the web side. Remember the memory tip: “Outbound from the talker, inbound on the listener, and stateful takes care of the return.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 company has a VPC with a CIDR of 172.16.0.0/16. They have a subnet 172.16.1.0/24 for web servers and another subnet 172.16.2.0/24 for database servers. The web servers need to access the database servers on port 3306. Which configuration is required?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Web SG: outbound to DB SG on port 3306. DB SG: inbound from web SG on port 3306.
Option C is correct because security groups are stateful and control traffic at the instance level. The web server security group needs an outbound rule allowing traffic to the database security group on port 3306, and the database security group needs an inbound rule allowing traffic from the web security group on port 3306. This ensures that only the web servers can initiate connections to the database servers on the required port.
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.
✗
Configure network ACLs to allow inbound on port 3306 from web subnet.
Why it's wrong here
NACLs are stateless and require explicit outbound rules; security groups are easier.
✗
Web SG: inbound from 0.0.0.0/0 on port 3306. DB SG: outbound to web SG on port 3306.
Why it's wrong here
Inbound on web is unnecessary.
✓
Web SG: outbound to DB SG on port 3306. DB SG: inbound from web SG on port 3306.
Why this is correct
Allows only MySQL traffic from web to DB.
Related concept
Read the scenario before looking for a memorised answer.
✗
Web SG: outbound to DB SG on all ports. DB SG: inbound from web SG on all ports.
Why it's wrong here
Allows all protocols, not least privilege.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the stateless nature of network ACLs with the stateful behavior of security groups, or they incorrectly assume that inbound rules on the source security group are needed instead of outbound rules on the source and inbound rules on the destination.
Detailed technical explanation
How to think about this question
Security groups act as a virtual firewall for EC2 instances, supporting allow rules only and being stateful, meaning that if you allow inbound traffic, the outbound return traffic is automatically permitted regardless of outbound rules. In contrast, network ACLs are stateless and require explicit inbound and outbound rules for each direction. When using security groups, you can reference other security groups as a source or destination, which simplifies rule management and ensures that only instances in the referenced group can communicate, as seen in the correct configuration where the DB SG references the Web SG as the source.
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.
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.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Web SG: outbound to DB SG on port 3306. DB SG: inbound from web SG on port 3306. — Option C is correct because security groups are stateful and control traffic at the instance level. The web server security group needs an outbound rule allowing traffic to the database security group on port 3306, and the database security group needs an inbound rule allowing traffic from the web security group on port 3306. This ensures that only the web servers can initiate connections to the database servers on the required port.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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