Quick Answer
The answer is that the security group only allows traffic from the 10.0.0.0/16 CIDR, which is the VPC’s internal range. This is why an EC2 instance at 10.0.1.50 can connect on port 443—it originates from within that allowed CIDR—while an on-premises client at 203.0.113.5 is blocked because its IP falls outside the permitted range. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of security group inbound rules and the implicit deny principle: security groups have no explicit deny rules, only an allow list, and any traffic not matching a rule is dropped. A common trap is assuming outbound rules or protocol mismatches are the issue, but here the inbound rule explicitly restricts source to the VPC CIDR. Memory tip: think of a security group as a bouncer checking a guest list—if your IP isn’t on the list (the VPC CIDR), you don’t get in, no matter how you knock.
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 security group is configured as shown. An EC2 instance in the same VPC with IP 10.0.1.50 can connect to the instance on port 443. An on-premises client with IP 203.0.113.5 cannot connect. What is the most likely reason?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The security group's outbound rules block return traffic
Why wrong: Outbound is allow all by default.
The protocol is incorrect; HTTPS uses UDP
Why wrong: HTTPS uses TCP.
The security group only allows traffic from the 10.0.0.0/16 CIDR
On-premises IP is not in that range.
The security group has a deny rule for the on-premises IP
Why wrong: No deny rule exists.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The security group only allows traffic from the 10.0.0.0/16 CIDR
Option D is correct because the security group only allows traffic from the 10.0.0.0/16 CIDR, which is the VPC range. On-premises traffic comes from outside that range. Option A is wrong because the protocol is correct. Option B is wrong because there is no explicit deny, only implicit deny. Option C is wrong because security groups do not have outbound rules shown, but outbound is allow all by default.
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
For each option: why learners choose it and why it is or isn't the right answer here.
The security group's outbound rules block return traffic
Why it's wrong here
Outbound is allow all by default.
The protocol is incorrect; HTTPS uses UDP
The security group only allows traffic from the 10.0.0.0/16 CIDR
Why this is correct
On-premises IP is not in that range.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
The security group has a deny rule for the on-premises IP
Why it's wrong here
No deny rule exists.
Common exam traps
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
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.
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
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
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
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
The correct answer is: The security group only allows traffic from the 10.0.0.0/16 CIDR — Option D is correct because the security group only allows traffic from the 10.0.0.0/16 CIDR, which is the VPC range. On-premises traffic comes from outside that range. Option A is wrong because the protocol is correct. Option B is wrong because there is no explicit deny, only implicit deny. Option C is wrong because security groups do not have outbound rules shown, but outbound is allow all by default.
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.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
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