CIDR blocks and IP addressing in AWS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a VPC with a CIDR block of 10.0.0.0/16.

AWS

resource "aws_vpc" "main" {
  cidr_block = "[1]"
}

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

A172.16.0.0/12

B192.168.0.0/24

C255.255.255.0

D10.0.0.0/16

Attempts:

3 left

💡 Hint

Common Mistakes

Using a subnet mask like 255.255.255.0 instead of a CIDR block.

Choosing a CIDR block that is too small for the VPC.

The CIDR block 10.0.0.0/16 defines a private network range suitable for a VPC.

2fill in blank

medium

Complete the code to create a subnet with CIDR block 10.0.1.0/24 inside the VPC.

AWS

resource "aws_subnet" "subnet1" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "[1]"
}

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

A10.0.1.0/24

B10.0.0.0/16

C192.168.1.0/24

D172.16.1.0/28

Attempts:

3 left

💡 Hint

Common Mistakes

Using a CIDR block outside the VPC range.

Choosing a subnet that overlaps with another subnet.

The subnet CIDR block 10.0.1.0/24 fits inside the VPC's 10.0.0.0/16 range.

3fill in blank

hard

Fix the error in the subnet CIDR block to be a valid subnet inside the VPC 10.0.0.0/16.

AWS

resource "aws_subnet" "subnet2" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "[1]"
}

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

A10.1.0.0/16

B192.168.0.0/24

C10.0.2.0/24

D172.16.0.0/12

Attempts:

3 left

💡 Hint

Common Mistakes

Using a subnet CIDR block that is the same size or larger than the VPC.

Choosing a subnet outside the VPC's IP range.

The subnet 10.0.2.0/24 is valid inside the VPC 10.0.0.0/16. The option 10.1.0.0/16 is outside the VPC range.

4fill in blank

hard

Fill both blanks to create a subnet with 512 IP addresses inside the VPC 10.0.0.0/16.

AWS

resource "aws_subnet" "subnet3" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "[1]"
  availability_zone = "[2]"
}

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

A10.0.4.0/23

Bus-east-1a

C10.0.8.0/24

Dus-west-2b

Attempts:

3 left

💡 Hint

Common Mistakes

Using a /24 subnet which only has 256 IP addresses.

Choosing an availability zone from a different region.

A /23 subnet has 512 IP addresses, and us-east-1a is a valid availability zone.

5fill in blank

hard

Fill all three blanks to define a route table with a route to the internet gateway for the VPC 10.0.0.0/16.

AWS

resource "aws_route_table" "rt" {
  vpc_id = aws_vpc.main.id

  route {
    cidr_block = "[1]"
    gateway_id = aws_internet_gateway.[2].id
    depends_on = [aws_internet_gateway.[3]]
  }
}

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

A0.0.0.0/0

Bigw_main

D10.0.0.0/16

Attempts:

3 left

💡 Hint

Common Mistakes

Using the VPC CIDR block instead of 0.0.0.0/0 for the route.

Mismatching the internet gateway resource name in gateway_id and depends_on.

The route to 0.0.0.0/0 sends all traffic to the internet gateway. The gateway_id and depends_on must reference the correct internet gateway resource name.

Practice

1. What does a CIDR block like 192.168.1.0/24 represent in AWS networking?

easy

A. A single IP address 192.168.1.24

B. An invalid IP address range

C. A subnet mask of 255.255.0.0

D. A range of IP addresses from 192.168.1.0 to 192.168.1.255

Solution

1. Step 1: Understand CIDR notation

   The number after the slash (/24) shows how many bits are fixed for the network part. Here, 24 bits fixed means the first 3 parts (192.168.1) are fixed.

2. Step 2: Calculate the IP range

   With 24 bits fixed, the last 8 bits can vary from 0 to 255, so the range is 192.168.1.0 to 192.168.1.255.

3. Final Answer:

   A range of IP addresses from 192.168.1.0 to 192.168.1.255 -> Option D

4. Quick Check:

   CIDR /24 means 256 addresses [OK]

Hint: Count bits after slash to find IP range size [OK]

Common Mistakes:

• Confusing CIDR with a single IP
• Misreading the subnet mask bits
• Assuming /24 means only 24 addresses

2. Which of the following is the correct CIDR notation for a subnet with 512 IP addresses?

easy

A. /23

B. /25

C. /22

D. /24

Solution

1. Step 1: Calculate bits needed for 512 addresses

   512 addresses require 9 bits (2^9 = 512) for host part.

2. Step 2: Determine CIDR prefix

   IPv4 has 32 bits total, so prefix = 32 - 9 = 23. So CIDR is /23.

3. Final Answer:

   /23 -> Option A

4. Quick Check:

   512 IPs = 2^(32-23) = 512 [OK]

Hint: Use 32 minus log2(IP count) for CIDR [OK]

Common Mistakes:

• Choosing /24 which gives only 256 addresses
• Confusing /22 with 1024 addresses
• Miscounting bits for hosts

3. Given the CIDR block 10.0.0.0/26, how many usable IP addresses are available for hosts?

medium

A. 64

B. 62

C. 32

D. 30

Solution

1. Step 1: Calculate total IPs in /26 block

   /26 means 32 - 26 = 6 bits for hosts, so total IPs = 2^6 = 64.

2. Step 2: Subtract network and broadcast addresses

   Two addresses are reserved (network and broadcast), so usable IPs = 64 - 2 = 62.

3. Final Answer:

   62 -> Option B

4. Quick Check:

   Usable IPs = total - 2 [OK]

Hint: Usable IPs = 2^(32 - prefix) - 2 [OK]

Common Mistakes:

• Counting all IPs as usable
• Forgetting to subtract network and broadcast
• Mixing up prefix length and host bits

4. You have a VPC with CIDR block 172.16.0.0/16. You want to create two subnets without overlapping IPs. Which pair of CIDR blocks is valid?

medium

A. 172.16.0.0/17 and 172.16.128.0/17

B. 172.16.0.0/18 and 172.16.64.0/17

C. 172.16.0.0/16 and 172.16.0.0/17

D. 172.16.0.0/15 and 172.16.128.0/17

Solution

1. Step 1: Understand the VPC range

   172.16.0.0/16 covers IPs from 172.16.0.0 to 172.16.255.255.

2. Step 2: Check subnet ranges for overlap

   /17 splits the /16 into two halves: 172.16.0.0 to 172.16.127.255 and 172.16.128.0 to 172.16.255.255. These do not overlap.

3. Final Answer:

   172.16.0.0/17 and 172.16.128.0/17 -> Option A

4. Quick Check:

   Non-overlapping halves split /16 into two /17s [OK]

Hint: Split CIDR by increasing prefix to avoid overlap [OK]

Common Mistakes:

• Choosing overlapping CIDRs
• Using larger CIDR than VPC block
• Ignoring subnet mask sizes

5. You need to design a VPC with exactly 3 subnets: one public with 100 IPs, one private with 50 IPs, and one isolated with 25 IPs. Which CIDR block allocation fits best inside 10.0.0.0/24 without overlap?

hard

A. 10.0.0.0/24, 10.0.1.0/25, 10.0.2.0/26

B. 10.0.0.0/26, 10.0.0.64/26, 10.0.0.128/26

C. 10.0.0.0/25, 10.0.0.128/26, 10.0.0.192/27

D. 10.0.0.0/26, 10.0.0.64/27, 10.0.0.96/28

Solution

1. Step 1: Calculate needed CIDR for each subnet

   100 IPs need at least /25 (128 IPs), 50 IPs need /26 (64 IPs), 25 IPs need /27 (32 IPs).

2. Step 2: Assign CIDRs inside 10.0.0.0/24 without overlap

   10.0.0.0/25 covers 0-127, 10.0.0.128/26 covers 128-191, 10.0.0.192/27 covers 192-223. These fit perfectly without overlap.

3. Final Answer:

   10.0.0.0/25, 10.0.0.128/26, 10.0.0.192/27 -> Option C

4. Quick Check:

   Subnet sizes fit and sum within /24 [OK]

Hint: Match subnet size to nearest CIDR block, assign sequentially [OK]

Common Mistakes:

• Using CIDRs too small for IP needs
• Overlapping subnet ranges
• Assigning subnets outside VPC CIDR