Multi-Tier VPC Design: Public and Private Subnets, NAT, IGW
A financial services company is designing a VPC with multiple tiers: web, application, and database. The web tier must be accessible from the internet, but the application and database tiers must not have direct internet access. The company needs to allow the application tier to download patches from the internet. Which THREE components should be included in the design? (Choose three.)
Quick Answer
The answer is a public subnet for the web tier, a NAT Gateway, and an Internet Gateway. This combination is correct because the Internet Gateway enables inbound traffic from the internet to the public subnet, while the NAT Gateway, placed in that public subnet, allows instances in private subnets—such as the application tier—to initiate outbound connections to download patches without exposing those private instances to inbound internet traffic. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of multi-tier VPC design with public and private subnets, specifically how to segment traffic while maintaining controlled outbound access. A common trap is confusing a NAT Gateway with an Egress-Only Internet Gateway, which only works for IPv6, or assuming a VPC Peering or Direct Connect can provide internet access—they cannot. Memory tip: think of the NAT Gateway as a "one-way door" for private subnets—outbound only, never inbound.
⚠ Common exam trap
The ANS-C01 exam often tests the misconception that a NAT Gateway alone provides internet access without an IGW, but the NAT Gateway must be placed in a public subnet that has a route to an IGW to function.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Internet Gateway (IGW)
An Internet Gateway (IGW) is required for the web tier to be accessible from the internet. It provides a target in the VPC route tables for internet-routable traffic and performs NAT for instances with public IPs. Without an IGW, the public subnet cannot communicate with the internet.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Direct Connect virtual interface
Why it's wrong here
Direct Connect provides private connectivity to on-premises, not internet access for patches.
- ✗
VPC Peering connection
Why it's wrong here
VPC Peering connects VPCs, not for internet access.
- ✓
Internet Gateway (IGW)
Why this is correct
IGW is required for the public subnet to receive internet traffic.
- ✓
NAT Gateway in a public subnet
Why this is correct
NAT Gateway allows instances in private subnets to initiate outbound internet traffic.
- ✓
Public subnet for the web tier
Why this is correct
A public subnet with a route to the IGW is needed for the web tier.
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Same concept, more angles
5 more ways this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company is designing a network for a multi-tier application that must meet compliance requirements. The architecture includes a VPC with public, private, and database subnets. Which TWO actions should be taken to ensure the database subnets are not directly accessible from the internet? (Select TWO.)
medium- A.Attach an Internet Gateway to the database subnets.
- B.Create a NAT Gateway in the database subnets.
- C.Associate a security group that denies inbound traffic from 0.0.0.0/0.
- ✓ D.Configure a network ACL on the database subnets to deny inbound traffic from 0.0.0.0/0.
- ✓ E.Place the database subnets in private subnets.
Why D: A network ACL (NACL) is a stateless firewall that operates at the subnet level. By configuring a NACL rule to deny inbound traffic from 0.0.0.0/0, you explicitly block all internet-originated traffic from reaching the database subnets, regardless of any security group rules. Option E is correct because placing the database subnets in private subnets means they have no direct route to an Internet Gateway, making them inherently inaccessible from the internet.
Variation 2. A company is designing a network for a multi-tier application. The web tier must be accessible from the internet, the application tier must be accessible only from the web tier, and the database tier must be accessible only from the application tier. Which architecture meets these requirements?
medium- A.Place each tier in a separate subnet and use network ACLs to allow traffic between tiers
- ✓ B.Place each tier in a separate subnet and use security groups to allow traffic between tiers
- C.Place all tiers in the same subnet and use security groups to control traffic
- D.Place all tiers in a public subnet and use a NAT gateway for the application and database tiers
Why B: Security groups act as a stateful virtual firewall at the instance level, allowing you to specify inbound and outbound rules based on source/destination security group IDs. By placing each tier in a separate subnet and referencing the web tier's security group as the source for the application tier's inbound rule, and the application tier's security group as the source for the database tier's inbound rule, you achieve the required least-privilege access without exposing the application or database tiers to the internet.
Variation 3. A company is designing a network for a multi-tier application. The web tier must be accessible from the internet, and the application tier must only be accessible from the web tier. Which architecture should they use?
medium- A.Web servers in private subnets with a NAT Gateway for outbound traffic, and application servers in public subnets
- ✓ B.Web servers in public subnets with an Internet Gateway, and application servers in private subnets with security groups allowing traffic only from the web tier
- C.All servers in a single VPC with VPC Peering to another VPC
- D.Web servers and application servers in public subnets, each with their own security group
Why B: It places the web servers in public subnets with an Internet Gateway (IGW) to allow direct inbound traffic from the internet, while the application servers reside in private subnets with security groups that explicitly permit traffic only from the web tier's security group. This ensures the application tier is not directly reachable from the internet, adhering to the principle of least privilege and defense in depth.
Variation 4. A company is designing a network for a three-tier application that must be PCI DSS compliant. The web tier must be accessible from the internet, the application tier must only be accessible from the web tier, and the database tier must only be accessible from the application tier. All tiers are in the same VPC. What is the MOST secure way to implement this?
easy- A.Use a VPN between the web and application tiers and between application and database tiers.
- B.Place all tiers in the same private subnet and use security groups for isolation.
- ✓ C.Place web tier in public subnets, application and database tiers in private subnets. Use security groups to allow only necessary traffic between tiers.
- D.Place all tiers in public subnets and use network ACLs to restrict traffic.
Why C: The most secure because it places the web tier in public subnets (for internet access) and the application and database tiers in private subnets (no direct internet access). Security groups are used to allow only the necessary traffic between tiers: from web to app on specific ports, and from app to db on specific ports. Security groups are stateful, which simplifies rule management and reduces attack surface. Option A is wrong because a VPN is unnecessary for intra-VPC traffic and adds complexity without improving security. Option B is wrong because placing all tiers in the same private subnet does not provide network isolation between tiers; security groups alone cannot prevent all lateral movement. Option D is wrong because placing all tiers in public subnets exposes the application and database tiers to the internet, and network ACLs are stateless, requiring symmetric rules and increasing management overhead.
Variation 5. A company is designing a network for a three-tier web application. The web tier must be accessible from the internet, while the application and database tiers must be in private subnets. The company wants to minimize the number of load balancers. Which design should be used?
easy- A.Place an internal Application Load Balancer in a private subnet and use a NAT gateway for internet access.
- B.Place an internet-facing Network Load Balancer in a public subnet and use it for all tiers.
- ✓ C.Place an internet-facing Application Load Balancer in a public subnet, web tier instances in public subnets, and app/database instances in private subnets.
- D.Place an internet-facing Application Load Balancer in a private subnet.
Why C: An internet-facing Application Load Balancer (ALB) in a public subnet can receive internet traffic and forward it to web tier instances in public subnets, while the application and database tiers remain in private subnets with no direct internet access. This design uses a single load balancer to handle all external traffic, minimizing the number of load balancers while maintaining security boundaries.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.