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Network Security, Compliance and Governance →mediumMultiple ChoiceObjective-mapped

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 is deploying a web application behind an Application Load Balancer (ALB). The application must only accept traffic from the ALB. Which security group configuration should be used for the EC2 instances?

A
Allow inbound from 0.0.0.0/0 on port 80
Why wrong: Incorrect. Allowing inbound from 0.0.0.0/0 on port 80 is insecure because it permits traffic from any source, not just the ALB.
B
Allow inbound from the ALB security group on port 80
Correct. Allowing inbound from the ALB security group on port 80 ensures that only traffic originating from the ALB is accepted, as the ALB's security group is automatically associated with all ALB nodes.
C
Allow inbound from the ALB's private IP addresses on port 80
Why wrong: Incorrect. Allowing inbound from the ALB's private IP addresses on port 80 is impractical because the ALB's IP addresses are dynamic and can change without notice, making it difficult to maintain an accurate rule.
D
Allow inbound from the VPC CIDR on port 80
Why wrong: Incorrect. Allowing inbound from the VPC CIDR on port 80 is too broad, as it would allow traffic from any resource within the VPC (e.g., other EC2 instances) that are not necessarily the ALB.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

Allow inbound from the ALB security group on port 80

The correct answer is Option B. To ensure the EC2 instances only accept traffic from the ALB, the security group should reference the ALB's security group as the source. This is a best practice because the ALB acts as a proxy, and using its security group dynamically allows traffic from any ALB instance, even if the ALB's IP addresses change. Option A is insecure as it allows traffic from any source (0.0.0.0/0). Option C is not recommended because the ALB's private IP addresses can change (e.g., during scaling or replacement), making static IP rules difficult to maintain. Option D is too broad, as it would allow traffic from any resource within the VPC CIDR, not just the ALB.

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.

✗
Allow inbound from 0.0.0.0/0 on port 80
Why it's wrong here
Incorrect. Allowing inbound from 0.0.0.0/0 on port 80 is insecure because it permits traffic from any source, not just the ALB.
✓
Allow inbound from the ALB security group on port 80
Why this is correct
Correct. Allowing inbound from the ALB security group on port 80 ensures that only traffic originating from the ALB is accepted, as the ALB's security group is automatically associated with all ALB nodes.
Related concept
CIDR notation defines the prefix length.
✗
Allow inbound from the ALB's private IP addresses on port 80
Why it's wrong here
Incorrect. Allowing inbound from the ALB's private IP addresses on port 80 is impractical because the ALB's IP addresses are dynamic and can change without notice, making it difficult to maintain an accurate rule.
✗
Allow inbound from the VPC CIDR on port 80
Why it's wrong here
Incorrect. Allowing inbound from the VPC CIDR on port 80 is too broad, as it would allow traffic from any resource within the VPC (e.g., other EC2 instances) that are not necessarily 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.

Visual reference

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.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

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: Allow inbound from the ALB security group on port 80 — The correct answer is Option B. To ensure the EC2 instances only accept traffic from the ALB, the security group should reference the ALB's security group as the source. This is a best practice because the ALB acts as a proxy, and using its security group dynamically allows traffic from any ALB instance, even if the ALB's IP addresses change. Option A is insecure as it allows traffic from any source (0.0.0.0/0). Option C is not recommended because the ALB's private IP addresses can change (e.g., during scaling or replacement), making static IP rules difficult to maintain. Option D is too broad, as it would allow traffic from any resource within the VPC CIDR, not just the ALB.

What should I do if I get this ANS-C01 question wrong?

What is the key concept behind this question?

CIDR notation defines the prefix length.

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