The answer is that the ALB subnets are missing a route to an internet gateway, which causes intermittent 503 errors. An internet-facing Application Load Balancer must have a default route (0.0.0.0/0) pointing to an internet gateway in its subnet’s route table; without it, the ALB cannot send responses back to clients over the internet, resulting in 503 errors even when the target group health checks show the web servers as healthy. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how ALB traffic flows and the critical distinction between public and private subnet routing requirements—a common trap is assuming the issue lies with the target group or cross-zone load balancing, but the intermittent nature points to a missing route, not a health check failure. Remember the memory tip: “No IGW route, no reply—503 is the ALB’s cry.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 has deployed a web application behind an Application Load Balancer (ALB) in a VPC. The ALB is in public subnets, and the web servers are in private subnets. The ALB is configured with a target group pointing to the web servers. Users report intermittent 503 errors. The web servers are healthy according to the target group health checks. What is the most likely cause?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ALB subnets do not have a route to an internet gateway
Option D is correct because internet-facing ALBs require a route to an internet gateway in their subnet route table to respond to clients. Option A is wrong because the ALB is internet-facing, not internal. Option B is wrong if the web servers are healthy. Option C is wrong because cross-zone load balancing is enabled by default and not likely intermittent.
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 ALB subnets do not have a route to an internet gateway
Why this is correct
Without a route to an internet gateway, the ALB cannot respond to client requests, causing 503 errors.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The target group health check interval is too long
Why it's wrong here
Health checks are passing, so the interval is not the issue.
✗
The ALB is configured as internal instead of internet-facing
Why it's wrong here
If it were internal, users would not be able to reach it at all, not just intermittent errors.
✗
Cross-zone load balancing is disabled
Why it's wrong here
Disabling cross-zone load balancing could cause uneven distribution but not 503 errors.
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 ALB subnets do not have a route to an internet gateway — Option D is correct because internet-facing ALBs require a route to an internet gateway in their subnet route table to respond to clients. Option A is wrong because the ALB is internet-facing, not internal. Option B is wrong if the web servers are healthy. Option C is wrong because cross-zone load balancing is enabled by default and not likely intermittent.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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