The answer is to set an inbound rule with the source set to sg-12345678. This is correct because security group referencing allows you to specify another security group as the source, meaning only traffic originating from instances associated with that specific security group is permitted, regardless of their IP addresses. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of stateful, logical traffic control versus CIDR-based rules; a common trap is confusing security group referencing with prefix lists or VPC CIDRs, which are broader and do not enforce instance-level granularity. Remember that security groups support logical references to other security groups within the same VPC or a peered VPC, making them ideal for micro-segmentation. Memory tip: think of it as a "group handshake" — only members of the referenced group can knock on the door.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 wants to restrict access to an EC2 instance such that only traffic from a specific security group (sg-12345678) can reach it. The instance is in a VPC with default network ACLs. What should the security group rule for the instance be?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Inbound rule with source set to sg-12345678
Option B is correct. Security groups allow you to reference another security group as a source. Option A is wrong because referencing a CIDR block would allow traffic from any instance in that CIDR, not specifically from the security group. Option C is wrong because security groups cannot reference prefixes. Option D is wrong because referencing a VPC CIDR is too broad.
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.
✗
Inbound rule with source set to the CIDR block of the VPC
Why it's wrong here
This would allow traffic from all instances in the VPC, not just the security group.
✓
Inbound rule with source set to sg-12345678
Why this is correct
Security groups can reference other security groups as a source.
Inbound rule with source set to the VPC's CIDR block
Why it's wrong here
Too broad; would allow traffic from any instance in the VPC.
✗
Inbound rule with source set to a prefix list that includes the security group
Why it's wrong here
Prefix lists are for IP CIDR blocks, not security groups.
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 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
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: Inbound rule with source set to sg-12345678 — Option B is correct. Security groups allow you to reference another security group as a source. Option A is wrong because referencing a CIDR block would allow traffic from any instance in that CIDR, not specifically from the security group. Option C is wrong because security groups cannot reference prefixes. Option D is wrong because referencing a VPC CIDR is too broad.
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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