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CIDR blocks and IP addressing in AWS - Cheat Sheet & Quick Revision

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Recall & Review
beginner
What does CIDR stand for and what is its purpose?
CIDR stands for Classless Inter-Domain Routing. It helps allocate IP addresses more efficiently by allowing flexible block sizes instead of fixed classes.
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beginner
Explain the format of a CIDR block, for example, 192.168.1.0/24.
A CIDR block has an IP address followed by a slash and a number. The number (/24) shows how many bits are fixed for the network part. Here, /24 means the first 24 bits are network, leaving 8 bits for hosts.
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intermediate
How many IP addresses are available in a /28 CIDR block?
A /28 block has 32 - 28 = 4 bits for hosts. That means 2^4 = 16 IP addresses total. Usually, 2 are reserved (network and broadcast), so 14 usable addresses.
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beginner
Why is CIDR important in AWS VPC networking?
CIDR lets you define the size of your VPC and subnets precisely. This helps avoid wasting IP addresses and allows better control over network design.
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beginner
What is the difference between public and private IP addresses in CIDR blocks?
Public IPs are reachable on the internet, while private IPs are used inside private networks. Private IP ranges are defined by specific CIDR blocks like 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.
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What does the '/16' mean in the CIDR block 10.0.0.0/16?
AIt is a subnet mask of 255.255.255.0
BThe last 16 bits are the network part
CThe first 16 bits are the network part
DThere are 16 IP addresses total
How many usable IP addresses are in a /30 subnet?
A6
B4
C8
D2
Which CIDR block is a private IP range?
A192.168.1.0/24
B8.8.8.0/24
C172.32.0.0/12
D100.64.0.0/10
Why should you avoid overlapping CIDR blocks in AWS VPCs?
AIt causes IP address conflicts and routing issues
BIt increases network speed
CIt reduces security
DIt automatically merges the networks
What is the subnet mask equivalent of /24?
A255.255.0.0
B255.255.255.0
C255.0.0.0
D255.255.255.255
Describe what a CIDR block is and how it helps in IP addressing.
Think about how IP addresses are grouped and counted.
You got /4 concepts.
    Explain why choosing the right CIDR block size is important when creating an AWS VPC.
    Consider how many devices you want to connect and how IPs are assigned.
    You got /4 concepts.

      Practice

      (1/5)
      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