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EC2 Basics

Links: 101 AWS SAA Index


  • It is a region scoped.
  • When you stop and start an instance its public IP will change but private IP remains same. If you want a fixed IP then you will have to use Elastic IPs.
  • When you start and stop an instance the underlying host may change. In general it is changed.
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  • vCPUs (virtual CPUs) are nothing but the number of threads. When select an EC2 instance you only see the vCPU.
  • Multiple threads can run on one vCPU (multithreading).
  • So if m5.2xlarge has 4 CPUs and 2 threads per CPU then we have a total of 8 vCPUs.
  • We can optimise the vCPU for cost savings. May be you need more RAM but less number of vCPUs or you don’t need multithreading for HPC workloads.
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  • There is a vCPU based limit on all types of EC2 Instance Purchasing Options per region. Like we can purchase only 20 reserved instances in a region. To increased this limit we have to request AWS.

Elastic IPs

  • We can attach Elastic IP to only one instance at a time.
  • We can only have 5 Elastic IP in our account.
  • Overall we should try and avoid using Elastic IP they reflect poor architecture.
  • For Elastic IPs you are charged as long as you have them and don’t use them. Once we provision an Elastic IP we need to associate it with an EC2 instance as soon as possible. Once you are done using the machine then disassociate the Elastic IP from the machine and then release it.
  • If you instance is in a stopped or terminated state and has an Elastic IP attached to it then you will be billed for it. If it is running you won't be billed for the Elastic IP.
So for elastic IP to not be billed three things have to be followed
  • The Elastic IP address is associated with an Amazon EC2 instance.
  • The instance associated with the Elastic IP address is running.
  • The instance has only one Elastic IP address attached to it.

EC2 User Data

  • Using EC2 User Data we can run a script when our instance boots for the first time.
  • We can run shell scripts and cloud init directives.
  • It is not run when rebooting instances but

    • You can update your configuration to ensure that your user data scripts and cloud-init directives run every time you restart your instance. By default, the scripts are run, only once during the boot process while first launching the instance.
  • It helps in automating boot tasks.

EC2 User Data script runs with the root user. Any command you have in there will have sudo rights.
You need to have the following at the beginning of you script #!/bin/bash

Instance Types

  • 7 types of instances

    • General Purpose: for everything. t2.micro is a general purpose instance.
    • Compute optimised: for HPC, gaming servers, dedicated web servers, ML.
    • Memory optimised: for in memory data bases like redis, real time processing of unstructured data (key value pairs.)
    • Storage optimised: for databases, OLTP systems, relational and non relational databases.
  • m5.2xlarge: Here m is the instance class. 5 is the generation which AWS upgrades over time and 2xlarge is the instance class size.

Security Groups

  • It is region scoped just like EC2.
  • Security groups act to firewall to EC2 instances.
  • Security groups are stateful. Once traffic goes in it will come out unaffected by the outbound rules.
  • Security groups can only contain allow rules. So you can only make the security groups more permissive.
  • A single security group can be attached to multiple instances and a single instance can have multiple security groups. Many to many relation.
  • Security Groups live outside of EC2 so if the traffic is blocked the EC2 instance won’t see it.
  • If your application is not accessible (time out) then it is a security group issue. If the application gives a connection refused error then it is an application error.
For a new security group all inbound traffic is blocked and all outbound traffic is allowed
For the default security group inbound traffic is allowed from the same SG and all outbound traffic is allowed.
  • So outbound rule is allowed in both the new and default security group.
  • You can delete the all outbound traffic allowed rule if you don’t want your instances to access the internet.
  • Using security group you create inbound/outbound rules based on port, protocol, and source/destination.
  • We can reference security groups in other security groups. Like EC2 instances can reference load balancer security group in the their inbound rules source. ^52df0e
  • Important ports
    • 22: SSH, SFTP
    • 21: FTP
    • 80, 443: Web traffic
    • 3389: RDP


  • Private key to access EC2 instances are also known as key pairs in questions.

Last updated: 2022-05-02