Public vs private subnets in AWS - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a public subnet with automatic public IP assignment.

AWS

resource "aws_subnet" "public" {
  vpc_id            = aws_vpc.main.id
  cidr_block        = "10.0.1.0/24"
  map_public_ip_on_launch = [1]
  availability_zone = "us-east-1a"
}

Drag options to blanks, or click blank then click option'

Atrue

B"true"

Cnull

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string "true" instead of boolean true

Setting map_public_ip_on_launch to false for a public subnet

Setting map_public_ip_on_launch to true enables automatic public IP assignment for instances in the subnet, making it public.

2fill in blank

medium

Complete the code to create a route in the public subnet's route table that directs internet traffic to the internet gateway.

AWS

resource "aws_route" "public_internet_access" {
  route_table_id         = aws_route_table.public.id
  destination_cidr_block = "0.0.0.0/0"
  [1]              = aws_internet_gateway.main.id
}

Drag options to blanks, or click blank then click option'

Anat_gateway_id

Binternet_gateway_id

Cgateway_id

Degress_gateway_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using nat_gateway_id instead of gateway_id for internet gateway

Using incorrect attribute names like internet_gateway_id

The correct attribute to specify the internet gateway in a route is gateway_id.

3fill in blank

hard

Fix the error in the private subnet route table to route internet traffic through the NAT gateway.

AWS

resource "aws_route" "private_nat_gateway" {
  route_table_id         = aws_route_table.private.id
  destination_cidr_block = "0.0.0.0/0"
  [1]              = aws_nat_gateway.main.id
}

Drag options to blanks, or click blank then click option'

Agateway_id

Bnat_gateway_id

Cinternet_gateway_id

Degress_gateway_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using gateway_id instead of nat_gateway_id for NAT gateway route

Confusing internet gateway with NAT gateway

For private subnets, internet traffic is routed through the NAT gateway using the nat_gateway_id attribute.

4fill in blank

hard

Fill both blanks to create a private subnet without automatic public IP assignment and associate it with the private route table.

AWS

resource "aws_subnet" "private" {
  vpc_id                  = aws_vpc.main.id
  cidr_block              = "10.0.2.0/24"
  map_public_ip_on_launch = [1]
  availability_zone       = "us-east-1b"
}

resource "aws_route_table_association" "private_assoc" {
  subnet_id      = aws_subnet.private.id
  route_table_id = [2]
}

Drag options to blanks, or click blank then click option'

Afalse

Btrue

Caws_route_table.private.id

Daws_route_table.public.id

Attempts:

3 left

💡 Hint

Common Mistakes

Setting map_public_ip_on_launch to true for private subnet

Associating private subnet with public route table

Private subnets do not assign public IPs automatically, so map_public_ip_on_launch is false. They are associated with the private route table.

5fill in blank

hard

Fill all three blanks to define a NAT gateway in a public subnet with an Elastic IP and associate it with the private route table.

AWS

resource "aws_eip" "nat_eip" {
  vpc = true
}

resource "aws_nat_gateway" "main" {
  allocation_id = aws_eip.nat_eip.id
  subnet_id     = [1]
}

resource "aws_route" "private_nat_route" {
  route_table_id         = [2]
  destination_cidr_block = "0.0.0.0/0"
  [3]              = aws_nat_gateway.main.id
}

Drag options to blanks, or click blank then click option'

Aaws_subnet.public.id

Baws_route_table.private.id

Cnat_gateway_id

Daws_subnet.private.id

Attempts:

3 left

💡 Hint

Common Mistakes

Placing NAT gateway in private subnet

Using public route table for private subnet route

Using gateway_id instead of nat_gateway_id for NAT gateway route

The NAT gateway must be in the public subnet (aws_subnet.public.id), the private route table is used for the route, and the route uses nat_gateway_id to point to the NAT gateway.

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