The answer is to configure an egress-only internet gateway in the VPC and add a route for ::/0 to that gateway in the private subnet’s route table. This works because an egress-only internet gateway is a horizontally scaled, redundant component that enables outbound IPv6 traffic from instances in a private subnet to the internet while preventing any inbound IPv6 connections initiated from the internet, effectively acting as the IPv6 counterpart to a NAT gateway for IPv4. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of asymmetric routing and the distinction between IPv4 and IPv6 egress patterns; a common trap is confusing the egress-only internet gateway with a standard internet gateway or assuming a NAT gateway supports IPv6. Remember the memory tip: “Egress-only = outbound-only for IPv6, just like NAT is outbound-only for IPv4.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 is designing a VPC with both IPv4 and IPv6 workloads. The VPC has an internet gateway, and the company wants to allow outbound IPv6 traffic to the internet from instances in a private subnet while blocking inbound IPv6 traffic from the internet. Which configuration should be used?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure an egress-only internet gateway in the VPC and add a route for ::/0 to the egress-only internet gateway in the private subnet's route table.
Option B is correct. An egress-only internet gateway allows outbound IPv6 traffic from instances in a private subnet to the internet, but prevents the internet from initiating connections to those instances. It is the IPv6 equivalent of a NAT gateway for IPv4. Option A is incorrect because a NAT gateway only supports IPv4. Option C is incorrect because a virtual private gateway is for VPN connections, not internet access. Option D is incorrect because a security group can control traffic but cannot provide outbound-only internet access for IPv6.
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.
✗
Configure a security group that allows outbound traffic and denies inbound traffic.
Why it's wrong here
Security groups control traffic but do not provide internet connectivity; a route to an internet gateway is needed.
✗
Configure a virtual private gateway and route traffic through it.
Why it's wrong here
Virtual private gateway is for VPN connections, not internet access.
✓
Configure an egress-only internet gateway in the VPC and add a route for ::/0 to the egress-only internet gateway in the private subnet's route table.
Why this is correct
Egress-only internet gateway provides outbound-only IPv6 internet access.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Configure an egress-only internet gateway in the VPC and add a route for ::/0 to the egress-only internet gateway in the private subnet's route table. — Option B is correct. An egress-only internet gateway allows outbound IPv6 traffic from instances in a private subnet to the internet, but prevents the internet from initiating connections to those instances. It is the IPv6 equivalent of a NAT gateway for IPv4. Option A is incorrect because a NAT gateway only supports IPv4. Option C is incorrect because a virtual private gateway is for VPN connections, not internet access. Option D is incorrect because a security group can control traffic but cannot provide outbound-only internet access for IPv6.
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.
About these practice questions
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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 has a VPC with an IPv4 CIDR block of 10.0.0.0/16. They need to add IPv6 connectivity to the VPC and allow resources in a private subnet to access the internet via IPv6. Which design should be used?
easy
A.Allocate an IPv6 CIDR block and add a route to an Internet Gateway from the private subnet.
B.Use a NAT instance with IPv6 support in a public subnet.
C.Allocate an IPv6 CIDR block and use a NAT Gateway for IPv6 traffic.
✓ D.Allocate an IPv6 CIDR block, create an egress-only Internet Gateway, and add a route to it from the private subnet.
Why D: An egress-only Internet Gateway (EIGW) is the correct AWS-managed service for enabling outbound-only IPv6 traffic from a VPC's private subnet to the internet, while preventing any unsolicited inbound connections. Unlike an Internet Gateway (IGW), which allows bidirectional traffic, an EIGW is specifically designed for IPv6 and ensures that resources in private subnets can initiate outbound connections without being directly reachable from the internet.
Last reviewed: Jun 20, 2026
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