This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 company has a VPC with multiple security groups. An EC2 instance in security group A needs to communicate with an RDS instance in security group B on port 3306. The security team wants to minimize exposure. What should the inbound rule in security group B be?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Allow inbound TCP 3306 from the CIDR block of the subnet where the EC2 instance resides
Why wrong: More permissive than necessary.
Allow inbound TCP 3306 from 0.0.0.0/0
Why wrong: Too permissive.
Allow inbound TCP 3306 from the private IP address of the EC2 instance
Why wrong: Not dynamic; fails if instance is replaced.
Allow inbound TCP 3306 from security group A
Precise and secure.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Allow inbound TCP 3306 from security group A
Option C is correct because referencing security group A as source allows traffic from any instance associated with security group A, which is the most specific and secure. Option A is wrong because it allows traffic from any instance in the VPC (0.0.0.0/0). Option B is wrong because it allows traffic from the entire subnet. Option D is wrong because it allows traffic from the instance's IP, but if the instance is replaced, the IP may change.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Allow inbound TCP 3306 from the CIDR block of the subnet where the EC2 instance resides
Why it's wrong here
More permissive than necessary.
Allow inbound TCP 3306 from 0.0.0.0/0
Why it's wrong here
Too permissive.
Allow inbound TCP 3306 from the private IP address of the EC2 instance
Why it's wrong here
Not dynamic; fails if instance is replaced.
Allow inbound TCP 3306 from security group A
Why this is correct
Precise and secure.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
Common exam traps
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
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
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
The correct answer is: Allow inbound TCP 3306 from security group A — Option C is correct because referencing security group A as source allows traffic from any instance associated with security group A, which is the most specific and secure. Option A is wrong because it allows traffic from any instance in the VPC (0.0.0.0/0). Option B is wrong because it allows traffic from the entire subnet. Option D is wrong because it allows traffic from the instance's IP, but if the instance is replaced, the IP may change.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Yes — watch for: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
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