Public Subnet vs Private Subnet - Routing and Internet Gateway - AWS Certification Cheat Sheet


Let’s get a quick overview of Public Subnet vs Private Subnet from an AWS certification perspective. We will look at important certification questions regarding Public Subnet vs Private Subnet.

You will learn

  • What are Public Subnets and Private Subnet?
  • When do we use Public Subnet vs Private Subnet?
  • How is Public Subnet different from Private Subnet?
  • What is an Internet Gateway?
  • Why is Internet Gateway Important?
  • What is Routing?
  • What are Route Tables?
  • How does Routing happen inside AWS?

AWS Certification Study Material and Notes - 25 PDF Cheat Sheets

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  • FAQs and Tutorials with 5-20 slides
  • Study Material and Notes to quickly review and prepare for certification exam
  • Certification Exam Tips
  • Certification and Interview Scenario Questions
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Public Subnet

In a Public Subnet:

  • Communication is allowed from subnet to internet
  • Communication is allowed from internet to subnet

Routing on the internet

Let’s first understand how routing works on the internet.

You have an IP address of a website you want to visit. There is no direct connection from your computer to the website.

Internet is actually a set of routers routing traffic. Each router has a set of rules that help it decide the path to the destination IP address.

Routing inside AWS

How does routing work inside AWS?

In AWS, route tables are used for routing. Route tables can be associated with VPCs and subnets. Each route table consists of a set of rules called routes:

  • Each route or routing rule has a destination and target
  • What CIDR blocks (range of addresses) should be routed to which target resource?
Destination Target
172.31.0.0/16 Local
0.0.0.0/0 igw-1234567

In the above configuration:

  • Rule 1 - Route requests to VPC CIDR 172.31.0.0/16 (172.31.0.0 to 172.31.255.255) to local resources within the VPC
  • Rule 2 - Route all other IP addresses (0.0.0.0/0) to internet (internet gateway)

Let’s consider a couple of examples:

What happens if I search for an address 172.31.0.10?

  • Two destinations match - 172.31.0.0/16 (172.31.0.0 to 172.31.255.255) and 0.0.0.0/0
  • The most specific rule wins. 172.31.0.0/16 is more specific
  • Result : Routing to a local resource

What happens if I search for an address 69.209.0.10?

  • One destination match - 69.208.0.10
  • Result : Routing to internet

Remember: The most specific matching route wins.

Public Subnet vs Private Subnet

How do public subnets connect to the internet?

Let’s examine the route table of a public subnet:

Name Destination Target Explanation
RULE 1 172.31.0.0/16 Local Local routing
RULE 2 0.0.0.0/0 igw-1234567 Internet routing

Any subnet which has a route to an internet gateway is called a public subnet.

An Internet Gateway enables internet communication for subnets.

Any subnet which DOES NOT have route to an internet gateway is called a private subnet

An Internet Gateway:

  • Sits between subnet (VPC) resources and internet
  • One to one mapping with a VPC
  • Supports IPv4 and IPv6
  • Translate private IP address to public IP address and vice-versa
  • DHCP option set: Assign custom host names and IP addresses to EC2 instances

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