Public vs private subnets in AWS - Performance Comparison

Time Complexity: Public vs private subnets

O(n)

Understanding Time Complexity

When working with public and private subnets, it's important to understand how the number of resources affects the time it takes to set up and manage network traffic.

We want to know how the time to configure and route traffic grows as we add more subnets.

Scenario Under Consideration

Analyze the time complexity of creating and routing traffic through public and private subnets.

// Create a VPC
aws ec2 create-vpc --cidr-block 10.0.0.0/16

// Create public subnets
for each public subnet:
  aws ec2 create-subnet --vpc-id vpc-123 --cidr-block 10.0.x.0/24
  aws ec2 create-route-table --vpc-id vpc-123
  aws ec2 associate-route-table --subnet-id subnet-public-x --route-table-id rtb-public-x
  aws ec2 create-route --route-table-id rtb-public-x --destination-cidr-block 0.0.0.0/0 --gateway-id igw-123

// Create private subnets
for each private subnet:
  aws ec2 create-subnet --vpc-id vpc-123 --cidr-block 10.0.y.0/24
  aws ec2 create-route-table --vpc-id vpc-123
  aws ec2 associate-route-table --subnet-id subnet-private-y --route-table-id rtb-private-y
  aws ec2 create-route --route-table-id rtb-private-y --destination-cidr-block 0.0.0.0/0 --nat-gateway-id nat-123

This sequence creates multiple public and private subnets, each with its own route table and routes to internet or NAT gateways.

Identify Repeating Operations

• Primary operation: Creating subnets and route tables, associating route tables, and creating routes.
• How many times: Once per subnet (public or private).

How Execution Grows With Input

Each new subnet requires creating a subnet, a route table, associating it, and adding routes. So, the work grows directly with the number of subnets.

Input Size (n)	Approx. Api Calls/Operations
10	About 40 (4 per subnet)
100	About 400
1000	About 4000

Pattern observation: The number of operations grows linearly as you add more subnets.

Final Time Complexity

Time Complexity: O(n)

This means the time to set up subnets and routing grows directly in proportion to how many subnets you create.

Common Mistake

[X] Wrong: "Adding more subnets won't increase setup time much because they share the same VPC and gateway."

[OK] Correct: Each subnet needs its own route table and associations, so the setup time increases with each subnet added.

Interview Connect

Understanding how subnet creation scales helps you design networks that grow smoothly and avoid surprises in setup time as your cloud grows.

Self-Check

"What if we used a single route table for all private subnets instead of one per subnet? How would the time complexity change?"

Practice

1. Which statement best describes a public subnet in AWS?

easy

A. It has direct internet access and assigns public IPs automatically.

B. It does not assign public IPs and blocks all internet traffic.

C. It is used only for internal communication within the VPC.

D. It requires a VPN connection to access the internet.

Solution

1. Step 1: Understand public subnet characteristics

   A public subnet allows resources to communicate directly with the internet by assigning public IP addresses.

2. Step 2: Compare with other subnet types

   Private subnets do not assign public IPs and restrict internet access, unlike public subnets.

3. Final Answer:

   It has direct internet access and assigns public IPs automatically. -> Option A

4. Quick Check:

   Public subnet = direct internet access [OK]

Hint: Public subnet = direct internet access + public IPs [OK]

Common Mistakes:

• Confusing private subnet with public subnet
• Thinking all subnets have internet access
• Assuming VPN is needed for public subnet

2. Which AWS subnet configuration correctly creates a private subnet?

easy

A. AssignPublicIpOnLaunch: true

B. EnableInternetGateway: true

C. MapPublicIpOnLaunch: false

D. RouteTable: 0.0.0.0/0 via IGW

Solution

1. Step 1: Identify private subnet IP assignment

   Private subnets do not assign public IPs automatically, so MapPublicIpOnLaunch must be false.

2. Step 2: Understand routing and gateway settings

   Internet Gateway (IGW) and routing to 0.0.0.0/0 are for public subnets, not private.

3. Final Answer:

   <code>MapPublicIpOnLaunch: false</code> -> Option C

4. Quick Check:

   Private subnet = no public IP assignment [OK]

Hint: Private subnet disables public IP assignment [OK]

Common Mistakes:

• Setting public IP assignment to true for private subnet
• Confusing internet gateway with subnet config
• Using public route table for private subnet

3. Given this route table for a subnet:
Destination: 0.0.0.0/0, Target: igw-12345
What type of subnet is this likely to be?

medium

A. Public subnet with internet access

B. Private subnet with no internet access

C. Isolated subnet with VPN only

D. Subnet with NAT gateway only

Solution

1. Step 1: Analyze route table destination and target

   The route directs all internet traffic (0.0.0.0/0) to an internet gateway (igw), which enables internet access.

2. Step 2: Determine subnet type from routing

   Routing to an internet gateway means the subnet is public and can access the internet directly.

3. Final Answer:

   Public subnet with internet access -> Option A

4. Quick Check:

   Route to IGW = public subnet [OK]

Hint: Route 0.0.0.0/0 to IGW means public subnet [OK]

Common Mistakes:

• Assuming NAT gateway means public subnet
• Confusing IGW with VPN or NAT
• Ignoring route table targets

4. You created a subnet with MapPublicIpOnLaunch: false but instances still get public IPs. What is the likely issue?

medium

A. The subnet is associated with a route table that routes to an internet gateway.

B. The instance launch configuration overrides subnet settings to assign public IPs.

C. The VPC has no internet gateway attached.

D. The subnet CIDR block overlaps with another subnet.

Solution

1. Step 1: Understand subnet vs instance IP assignment

   Subnet setting disables automatic public IP assignment, but instance launch settings can override this.

2. Step 2: Identify cause of public IP despite subnet config

   If instance launch config explicitly assigns public IPs, it overrides subnet default behavior.

3. Final Answer:

   The instance launch configuration overrides subnet settings to assign public IPs. -> Option B

4. Quick Check:

   Instance config can override subnet IP assignment [OK]

Hint: Instance launch config can override subnet IP settings [OK]

Common Mistakes:

• Blaming route table for IP assignment
• Ignoring instance-level settings
• Confusing CIDR overlap with IP assignment

5. You want to host a web server accessible from the internet and a database only accessible internally. How should you design your subnets?

hard

A. Place both web server and database in private subnets and use a VPN for access.

B. Place both web server and database in a public subnet with security groups restricting access.

C. Place the web server in a private subnet and the database in a public subnet.

D. Place the web server in a public subnet and the database in a private subnet.

Solution

1. Step 1: Identify subnet roles for internet access

   The web server needs internet access, so it belongs in a public subnet with a route to the internet gateway.

2. Step 2: Secure the database internally

   The database should be in a private subnet without direct internet access to keep it secure.

3. Final Answer:

   Place the web server in a public subnet and the database in a private subnet. -> Option D

4. Quick Check:

   Internet-facing resources in public, internal in private [OK]

Hint: Web server public, database private subnet [OK]

Common Mistakes:

• Putting database in public subnet
• Putting web server in private subnet
• Relying only on security groups without subnet design