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 has a VPC with an Application Load Balancer (ALB) in front of a fleet of EC2 instances. The security group for the EC2 instances must allow traffic only from the ALB. Which source should be specified in the security group inbound rule?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ID of the ALB's security group
Option B is correct because the ALB's security group ID can be used as the source in the instance security group, allowing traffic only from the ALB. Option A is wrong because the ALB's public IPs can change; using security group IDs is more reliable. Option C is wrong because the VPC CIDR is too broad. Option D is wrong because the subnet CIDR of the ALB is also too broad and not as secure.
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.
✗
The ALB's public IP address
Why it's wrong here
ALB public IPs can change, and using IPs is less flexible.
✗
The subnet CIDR block of the ALB's subnets
Why it's wrong here
This allows traffic from all instances in those subnets, not just the ALB.
✓
The ID of the ALB's security group
Why this is correct
Referencing the ALB's security group as a source ensures only traffic from the ALB is allowed.
This allows traffic from any resource in the VPC, not just the ALB.
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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The ID of the ALB's security group — Option B is correct because the ALB's security group ID can be used as the source in the instance security group, allowing traffic only from the ALB. Option A is wrong because the ALB's public IPs can change; using security group IDs is more reliable. Option C is wrong because the VPC CIDR is too broad. Option D is wrong because the subnet CIDR of the ALB is also too broad and not as secure.
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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