The answer is to create a private subnet for the DMS replication instance and attach a NAT gateway to allow outbound traffic to the on-premises database. This resolves the connection error because the DMS replication instance, when placed in a public subnet, lacks a route to the on-premises network through the firewall; a private subnet with a NAT gateway provides the necessary outbound connectivity for AWS DMS source database firewall connectivity, enabling the instance to initiate traffic to the on-premises database while remaining isolated from the internet. On the AWS Certified SAP on AWS Specialty PAS-C01 exam, this scenario tests your understanding of hybrid network architectures, specifically how to troubleshoot DMS connectivity when the source is behind a firewall and Direct Connect is in use. A common trap is assuming that opening firewall ports alone is sufficient, but the DMS instance must also have a valid network path—often via a NAT gateway or VPC peering—to reach the source. Memory tip: think of the NAT gateway as the “outbound pass” for DMS to talk through the firewall.
PAS-C01 Migration Practice Question
This PAS-C01 practice question tests your understanding of migration. 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 manufacturing company is migrating its SAP ECC system to AWS. The system includes an SAP HANA database (1 TB) and an SAP NetWeaver application server. They plan to use AWS Database Migration Service (DMS) for the database and AWS Application Migration Service (MGN) for the application server. The network connection is AWS Direct Connect with 1 Gbps bandwidth. During a test migration, the DMS task for the database fails with a connection error. The source database is on an on-premises server that is behind a firewall. The administrator has already opened the necessary ports in the firewall. The DMS replication instance is in a VPC with a public subnet. Which step should the administrator take to resolve the connection issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a private subnet for the DMS replication instance and attach a NAT gateway to allow outbound traffic to the on-premises database.
DMS replication instance needs network connectivity to the source database. If the source is behind a firewall, the DMS instance may need to be in a private subnet with a NATgateway or VPC peering. Option A is correct because creating a private subnet and enabling NAT gateway allows outbound connectivity. Option B is wrong because DMS endpoint is for configuring source/target, not for network connectivity. Option C is wrong because the firewall is already open. Option D is wrong because modifying the source database configuration is not needed.
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.
✗
Modify the DMS source endpoint to use a different port.
Why it's wrong here
Port is not the issue.
✓
Create a private subnet for the DMS replication instance and attach a NAT gateway to allow outbound traffic to the on-premises database.
Why this is correct
Private subnet with NATgateway enables connectivity to on-premises.
Open additional ports in the on-premises firewall.
Why it's wrong here
Ports are already open.
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 PAS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Migration — This question tests Migration — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Create a private subnet for the DMS replication instance and attach a NAT gateway to allow outbound traffic to the on-premises database. — DMS replication instance needs network connectivity to the source database. If the source is behind a firewall, the DMS instance may need to be in a private subnet with a NAT gateway or VPC peering. Option A is correct because creating a private subnet and enabling NAT gateway allows outbound connectivity. Option B is wrong because DMS endpoint is for configuring source/target, not for network connectivity. Option C is wrong because the firewall is already open. Option D is wrong because modifying the source database configuration is not needed.
What should I do if I get this PAS-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 PAS-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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