CIDR blocks and IP addressing in AWS - Step-by-Step Execution

Process Flow - CIDR blocks and IP addressing

Start with IP address

↓

Apply CIDR mask

↓

Calculate network address

↓

Calculate range of IPs

↓

Identify usable IPs

↓

Assign IPs to devices

↓

End

This flow shows how an IP address and CIDR mask combine to define a network and usable IP range.

Execution Sample

AWS

IP = 192.168.1.0/29
# Calculate network and usable IPs

This example calculates the network and usable IP addresses for the CIDR block 192.168.1.0/29.

Process Table

Step	Action	Calculation/Condition	Result
1	Start with IP and CIDR	IP=192.168.1.0, Mask=/29	Given IP block
2	Calculate subnet mask	/29 means 29 bits for network	Subnet mask = 255.255.255.248
3	Calculate number of IPs	2^(32-29) = 8	Total IPs = 8
4	Calculate network address	IP & subnet mask	192.168.1.0
5	Calculate broadcast address	Network + total IPs - 1	192.168.1.7
6	Identify usable IPs	Exclude network and broadcast	192.168.1.1 to 192.168.1.6
7	Assign IPs to devices	Use usable IP range	Devices get IPs 192.168.1.1-192.168.1.6
8	End	All usable IPs assigned	Finished

💡 All usable IPs assigned, network and broadcast reserved

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
IP	192.168.1.0/29	192.168.1.0/29	192.168.1.0/29	192.168.1.0	192.168.1.0	192.168.1.0	192.168.1.0
Subnet Mask	N/A	255.255.255.248	255.255.255.248	255.255.255.248	255.255.255.248	255.255.255.248	255.255.255.248
Total IPs	N/A	N/A	8	8	8	8	8
Network Address	N/A	N/A	N/A	192.168.1.0	192.168.1.0	192.168.1.0	192.168.1.0
Broadcast Address	N/A	N/A	N/A	N/A	192.168.1.7	192.168.1.7	192.168.1.7
Usable IP Range	N/A	N/A	N/A	N/A	N/A	192.168.1.1-192.168.1.6	192.168.1.1-192.168.1.6

Key Moments - 3 Insights

Why do we exclude the first and last IP addresses in the range?

The first IP is the network address and the last is the broadcast address, both reserved and not assignable to devices (see execution_table steps 4 and 5).

How do we calculate the total number of IPs from the CIDR mask?

Subtract the CIDR number from 32, then raise 2 to that power (2^(32 - CIDR)). For /29, 2^(32-29) = 8 (see execution_table step 3).

What does the subnet mask 255.255.255.248 mean in binary?

It means the first 29 bits are network bits (ones), and the last 3 bits are host bits (zeros), defining the size of the subnet (see execution_table step 2).

Visual Quiz - 3 Questions

Look at the execution table, what is the broadcast address calculated at step 5?

A 192.168.1.0

B 192.168.1.6

C 192.168.1.7

D 192.168.1.1

💡 Hint

Check the 'Result' column at step 5 in the execution_table.

At which step do we find the number of total IP addresses in the block?

A Step 3

B Step 2

C Step 4

D Step 6

💡 Hint

Look for the calculation of 2^(32 - CIDR) in the execution_table.

If the CIDR was changed to /30, how would the total IPs change in the execution table?

A Total IPs would be 8

B Total IPs would be 4

C Total IPs would be 16

D Total IPs would be 2

💡 Hint

Recall the formula 2^(32 - CIDR) and apply it for /30.

Concept Snapshot

CIDR blocks define IP ranges using a mask.
CIDR /n means n bits for network, rest for hosts.
Total IPs = 2^(32 - n).
First IP = network address (not usable).
Last IP = broadcast address (not usable).
Usable IPs lie between these two.

Full Transcript

CIDR blocks combine an IP address with a mask to define a network range. The mask tells how many bits are for the network part. For example, /29 means 29 bits for network and 3 bits for hosts. We calculate total IPs by 2 raised to the power of (32 minus the mask). The first IP is the network address and the last is the broadcast address; both are reserved and cannot be assigned to devices. Usable IPs are those in between. This process helps assign IPs efficiently in cloud networks.

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