I am currently setting up a custom Virtual Private Cloud (VPC) on AWS and I am confused about the routing components. I understand both allow for internet connectivity, but I don't quite grasp when to use one over the other. Could someone explain the specific architectural differences between an Internet Gateway and a NAT Gateway? Specifically, how do they handle inbound versus outbound traffic for my public and private subnets to ensure the environment remains secure?
3 answers
The primary difference lies in the direction of connectivity and the type of subnet they serve. An Internet Gateway (IGW) is a horizontally scaled, redundant, and highly available VPC component that allows communication between your VPC and the internet. It supports both inbound and outbound traffic, which is why it's used for public subnets where your web servers live. On the other hand, a NAT Gateway is used specifically for private subnets. it allows instances in those private subnets to connect to the internet for software updates (outbound) but prevents the internet from initiating a connection with those instances (inbound). This provides an essential layer of security for your backend databases and application servers.
When you are configuring your route tables, are you planning to place the NAT Gateway in the public or the private subnet itself? I ask because many developers make the mistake of placing the NAT Gateway in a private subnet, which actually prevents it from reaching the Internet Gateway and causes the entire routing logic to fail for those backend instances.
Think of an IGW as a two-way door for public traffic and a NAT Gateway as a one-way mirror. The NAT lets your private servers see out, but the world can't see in.
I love that analogy, Karen! It perfectly summarizes the security aspect. To add to that, remember that NAT Gateways incur hourly costs and data transfer fees, whereas an Internet Gateway is a managed service that is free of charge in your AWS account.
Kevin, that is a great catch. To answer your question, I was planning on putting it in the public subnet because I read that the NAT Gateway itself needs a public IP address (EIP) to function. Does this mean my private instances will then use the NAT Gateway's public IP for their outbound requests, effectively masking their internal private IP addresses from the external web?