The correct choice is to configure a security group with an inbound rule allowing traffic from the on-premises CIDR and an outbound rule allowing traffic to 0.0.0.0/0. This works because security groups are stateful, meaning that when you permit outbound traffic to the internet, the corresponding return traffic is automatically allowed, even without an explicit inbound rule for that response. For on-premises to internet traffic, the stateful nature of security groups handles the bidirectional flow seamlessly, while a network ACL would require separate inbound and outbound rules for each direction, making it error-prone for this scenario. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of stateful versus stateless filtering in hybrid connectivity setups, often appearing as a trap where candidates mistakenly choose a NACL due to its familiarity with subnet-level control. A key memory tip is to remember that security groups are like a smart bouncer who remembers who they let out, while NACLs are like a checklist that needs every step written down.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 uses AWS Direct Connect to connect its data center to a VPC. The security team wants to ensure that only the on-premises network can initiate connections to EC2 instances in the VPC, but the EC2 instances should be allowed to initiate outbound connections to the internet. Which configuration should be implemented?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure a security group with inbound allow from on-premises CIDR and outbound allow to 0.0.0.0/0
Security groups are stateful, so allowing inbound from on-premises and outbound to 0.0.0.0/0 works. Option B is correct because security groups automatically allow return traffic. Option A is wrong because NACLs are stateless and require explicit rules for return traffic. Option C is wrong because it would block all inbound. Option D is wrong because it would allow all inbound.
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
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Configure a network ACL with inbound allow from on-premises CIDR and outbound allow to 0.0.0.0/0
Why it's wrong here
NACLs are stateless and require separate outbound allow for ephemeral ports.
✓
Configure a security group with inbound allow from on-premises CIDR and outbound allow to 0.0.0.0/0
Why this is correct
Security groups are stateful and track connections.
Configure a network ACL with inbound allow from 0.0.0.0/0 and outbound allow to 0.0.0.0/0
Why it's wrong here
This would allow all inbound traffic, violating the requirement.
✗
Configure a security group with inbound deny from 0.0.0.0/0 and outbound allow to 0.0.0.0/0
Why it's wrong here
This would block inbound from on-premises as well.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
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
How to think about this question
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.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
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
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
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.
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Configure a security group with inbound allow from on-premises CIDR and outbound allow to 0.0.0.0/0 — Security groups are stateful, so allowing inbound from on-premises and outbound to 0.0.0.0/0 works. Option B is correct because security groups automatically allow return traffic. Option A is wrong because NACLs are stateless and require explicit rules for return traffic. Option C is wrong because it would block all inbound. Option D is wrong because it would allow all inbound.
What should I do if I get this ANS-C01 question wrong?
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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