This ANS-C01 practice question tests your understanding of network implementation. 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 is designing a multi-tier application with web servers in public subnets and database servers in private subnets. The database servers should only be accessible from the web servers. Which AWS feature should be used to enforce this?
Security groups that reference the web server security group as a source
Security groups allow you to specify another security group as the source, enabling fine-grained access control.
B
VPC endpoints to restrict access to the database servers
Why wrong: VPC endpoints are for accessing AWS services, not for controlling traffic between instances.
C
Network ACLs with allow rules for the web server subnet CIDR
Why wrong: NACLs are stateless and require both inbound and outbound rules; they also apply to all instances in the subnet, not just the database servers.
D
Network ACLs with deny rules for all traffic except from the web server subnet
Why wrong: NACLs are stateless and less secure than security groups for this purpose.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Security groups that reference the web server security group as a source
Option C is correct because security groups act as a virtual firewall for instances, and you can allow inbound traffic from a specific security group (the web servers' security group). Option A is wrong because NACLs are stateless and applied at subnet level, not instance-level. Option B is wrong because network ACLs are not as granular as security groups for this use case. Option D is wrong because VPC endpoints are for accessing AWS services privately.
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.
✓
Security groups that reference the web server security group as a source
Why this is correct
Security groups allow you to specify another security group as the source, enabling fine-grained access control.
VPC endpoints to restrict access to the database servers
Why it's wrong here
VPC endpoints are for accessing AWS services, not for controlling traffic between instances.
✗
Network ACLs with allow rules for the web server subnet CIDR
Why it's wrong here
NACLs are stateless and require both inbound and outbound rules; they also apply to all instances in the subnet, not just the database servers.
✗
Network ACLs with deny rules for all traffic except from the web server subnet
Why it's wrong here
NACLs are stateless and less secure than security groups for this purpose.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Security groups that reference the web server security group as a source — Option C is correct because security groups act as a virtual firewall for instances, and you can allow inbound traffic from a specific security group (the web servers' security group). Option A is wrong because NACLs are stateless and applied at subnet level, not instance-level. Option B is wrong because network ACLs are not as granular as security groups for this use case. Option D is wrong because VPC endpoints are for accessing AWS services privately.
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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