CIDR block (Classless Inter-Domain Routing) interpretation

by XDK 4. August 2019 06:09


CIDR (Classless Inter-Domain Routing) notation is a compact representation of an IP address and its associated routing prefix. The notation is constructed from an IP address, a slash ('/') character, and a decimal number. The number is the count of leading 1 bits in the subnet mask. Larger values here indicate smaller networks. CIDR was developed as an alternative to traditional subnetting. The idea is that you can add a specification in the IP address itself as to the number of significant bits that make up the routing or networking portion.

IPv4 CIDR blocks interpretation :

IPv4 : a.b.c.d
Bits : --> 32 bits

CIDR block Notation : a.b.c.d/prefix where prefix is the number of IP address the network can have.

Example :

IPs calculation for the above CIDR block will be [192(8bits).168(8bits).100(8bits).0(8bits)] = 2^32-22 = 2^10 = 1,024
where 32 -> 8 * 4 bits , 22 -> prefix

Total number of IPs for CIDR block will be 1,024 -> 255 -> 255 -> 255 -> 255
where 0 -> 255 = 256 IPs

Total IPs = 256 * 4 = 1,024

By default, AWS uses 5 IPs out of these 256 IPs, So the number of IPs released will be 256 - 5 = 251

IPs used by AWS:

0 -> Network Address
1 -> Amazon VPC Routing
2 -> Domain Name System (DNS)
3 -> Future use by AWS
255 -> Broadcasting

In AWS, a subnet can have 1 primary CIDR block and 4 secondary CIDR blocks


Amazon Web Service

What is the difference between Security Group and Network ACL in AWS?

by XDK 3. August 2019 19:04


 Security Group vs Network ACL (Network Access Control List)


Amazon Web Service

Shell Scripting with AWS Command Line Interface

by XDK 3. August 2019 00:12


1) List of EC2 instance IDs:
aws ec2 describe-instances | grep InstanceId | tr '",' ' '

2) List of Load Balancers by Name:
aws elbv2 describe-load-balancers | grep LoadBalancerName | tr '",' ' '

3) List of Target Groups by Name:
aws elbv2 describe-target-groups | grep TargetGroupName | tr '",' ' '

4) List of Autoscale Groups by Name:
aws autoscaling describe-auto-scaling-groups | grep AutoScalingGroupName | tr '",' ' '

5) List of Launch configurations by Name:
aws autoscaling describe-launch-configurations | grep LaunchConfigurationName | tr '",' ' '

 6) List of available IAM users by Name:
aws iam list-users | grep UserName | tr '",' ' '


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Amazon Web Service

How do I run .sh files in Linux?

by XDK 3. August 2019 00:07


The procedure to run the .sh file shell script on Linux is as follows:

Set execute permission on your script:
chmod +x <script-name>.sh

To run your script, enter:
./<script-name>.sh OR sh <script-name>.sh OR bash script-name>.sh

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Amazon Web Service

User-data bootstrap scripts is not running on the Amazon Linux AMI

by XDK 1. August 2019 05:43

Quick fix:

Adding "#cloud-boothook" command to the script fixes the issue.


echo 'User-data bootstrap script invoked' > /home/ec2-user/bootstrap-script-output.txt


Amazon Web Service

About the author

My name is Xavier Dilip Kumar Jayaraj having 16+ years of IT experience which includes solid experience and depth Knowledge in Application Life Cycle Management, Configuration Management, Implementation and Support using TFS on-premises and Azure DevOps. I have invested in gaining DevOps knowledege to expertise with Cloud Computing providers namely Microsoft Azure and Amazon Web Services in recent years. I am very positive to learn and adapt emerging technologies to client’s environment.

Microsoft Certified: Azure Administrator Associate

Microsoft Certified: Azure DevOps Engineer Expert

DevOps Engineer Certificate Program | Transcript 

OTP-AWSD5: AWS IoT: Developing and Deploying an Internet of Things

[PCEP-30-01] PCEP – Certified Entry-Level Python Programmer

Quotes I Like

"Failure will never overtake me if my determination to succeed is strong enough."  - Dr. APJ. Abdul Kalam

"Always be yourself, express yourself, have faith in yourself, do not go out and look for a successful personality and duplicate it." - Bruce Lee

"Technology is just a tool. In terms of getting the kids working together and motivating them, the teacher is the most important." - Bill Gates

"Innovation distinguishes between a leader and a follower." - Steve Jobs



The information provided here is based on my expreriences, troubleshooting and online/offline findings. It can be used as is on your own risk without any warranties and I impose no rights.