The answer is the external server's firewall blocking traffic from the NAT Gateway's Elastic IP address. When a private EC2 instance initiates outbound SSH through a NAT Gateway, the gateway translates the private IP to its Elastic IP, so the external server sees the source as that public IP. If the external server's firewall or security group does not explicitly allow inbound SSH from that Elastic IP, the connection is dropped, even though the VPC's security group and NACL are correctly configured. This scenario tests your understanding of how NAT Gateway source IP translation interacts with external firewalls, a common trap on the AWS Certified Advanced Networking Specialty ANS-C01 exam where candidates mistakenly blame the NAT Gateway or route tables. Remember the key insight: the NAT Gateway is a transparent router, not a firewall—it never blocks traffic itself. Memory tip: "NAT hides, firewall decides"—the NAT Gateway hides the private IP, but the external firewall decides whether to accept the translated public IP.
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 has a VPC with a CIDR of 10.0.0.0/16. They have two subnets: 10.0.1.0/24 (public) and 10.0.2.0/24 (private). They launch a NAT Gateway in the public subnet and add a route in the private subnet route table: destination 0.0.0.0/0, target nat-gateway-id. An EC2 instance in the private subnet can ping an external server, but cannot connect to it via SSH. The security group allows outbound SSH (port 22), and the NACL allows outbound ephemeral ports. What is the likely cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The external server's firewall is blocking traffic from the NAT Gateway's Elastic IP address.
The issue is likely that the external server's security group or firewall is blocking inbound SSH from the NATGateway's public IP. The NAT Gateway uses its Elastic IP for outbound traffic, and the external server must allow that IP. Option B is wrong because the NAT Gateway does not block SSH. Option C is wrong because the NACL is permissive. Option D is wrong because the route is correct.
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 NACL on the private subnet is blocking outbound ephemeral ports.
Why it's wrong here
Ping works, so outbound is allowed.
✗
The route table in the private subnet does not have a route to the NAT Gateway.
Why it's wrong here
Ping works, so routing is correct.
✓
The external server's firewall is blocking traffic from the NAT Gateway's Elastic IP address.
Why this is correct
The external server must allow the NATGateway's public IP on port 22.
The NAT Gateway is not configured to allow SSH traffic.
Why it's wrong here
NATGateway is transparent to protocols; it forwards any TCP/UDP traffic.
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 external server's firewall is blocking traffic from the NAT Gateway's Elastic IP address. — The issue is likely that the external server's security group or firewall is blocking inbound SSH from the NAT Gateway's public IP. The NAT Gateway uses its Elastic IP for outbound traffic, and the external server must allow that IP. Option B is wrong because the NAT Gateway does not block SSH. Option C is wrong because the NACL is permissive. Option D is wrong because the route is correct.
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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