Allowing Outbound IPv6 Traffic While Blocking Inbound: Egress-Only Internet Gateway
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?
Quick Answer
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.”
⚠ Common exam trap
The ANS-C01 exam often tests the misconception that a NAT gateway can handle IPv6 traffic, but NAT is not used with IPv6 by design; the correct solution is an egress-only internet gateway, which is a distinct AWS resource specifically for IPv6 outbound-only access.
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
✓
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.
An egress-only internet gateway (EIGW) is a horizontally scaled, redundant component that allows outbound IPv6 traffic from instances in a private subnet to the internet, but prevents the internet from initiating an inbound IPv6 connection to those instances. By adding a default route (::/0) pointing to the EIGW in the private subnet's route table, outbound IPv6 traffic is forwarded to the internet gateway, while the EIGW inherently drops any unsolicited inbound IPv6 traffic. This meets the requirement exactly, as security groups and network ACLs alone cannot fully block inbound IPv6 from the internet when a standard internet gateway is used.
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.
- ✗
Configure a NAT gateway in a public subnet.
Why it's wrong here
NAT gateways only support IPv4 traffic.
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1 more way this is tested on ANS-C01
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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.
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.