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VPC & Subnets

Links: 101 AWS SAA Index


  • VPC is a regional resource.
  • When you use your AWS account you have a default VPC in each region with a public subnet in each AZ but no private subnets.
  • Amazon VPC enables you to launch AWS resources into a virtual network that you've defined. This virtual network closely resembles a traditional network that you'd operate in your own data centre, with the benefits of using the scalable infrastructure of AWS.
  • Amazon VPC is the networking layer for Amazon EC2.
  • VPC is logically isolated from other virtual networks in the AWS Cloud.
  • We can have multiple VPCs in an AWS region

    • 5 per region is the soft limit
  • Max CIDR per VPC is 5

    • Min size is /28 (16 IP addresses)
    • Max size is /16 (65536 IP addresses)
  • VPC is private and only private IP ranges are allowed.

    • Class A: to
    • Class B: to
    • Class C: to
Your VPC CIDR should not overlap with your other networks.

Also it should be in the CIDR range of 16 to 28.

Default VPC

  • The default VPC has a default subnet in every AZ.
  • All new accounts get a default VPC
  • New EC2 instances are launched in the default VPC if no subnet is specified.
  • Default VPC has internet connectivity and all EC2 instances inside it will have public IPv4 addresses.
  • We also get a public and private IPv4 DNS name for the EC2 instances.
  • We are given a default VPC because if we don’t have one it will be very difficult for the new comers.


  • Once the VPC is created next step is to add subnets to the VPC.

  • AWS reserves 5 addresses in a subnet (first 4 and last 1). These addresses are not available for use and cannot be assigned to EC2 instances. Example

    • Network Address
    • reserved by AWS for the VPC router
    • reserved by AWS for mapping to Amazon-provided DNS
    • reserved by AWS for future use
    • Network Broadcast Address.
  • 3 are reserved by AWS and 2 are in general reserved by any network. Router can also be thought of as general because any network will have it so mainly 2 by AWS.

If you asked you need x IP address and what should be the CIDR range always remember 5 IP addresses are reserved.

If you need 29 IP addresses then you need a CIDR of /26.

  • A VPC spans all the AZs in the region but a subnet must reside entirely within one Availability Zone and cannot span zones
There will be questions asking how many subnets should be created
  • Always remember you have to atleast create 2 subnets in an AZ. One private and one public and for HA you need atleast 2 AZs. So minimum total of 4 subnets.
  • There can be more and this depends on the application structure described by the question. If the question demands that the DB should be in a different private subnet from the application layer private subnet then there will be a total of 6 subnets required for HA.

Why subnets and VPC

  • At a high level, you can think of a VPC in AWS as a logical container that separates resources you create from other customers within the Amazon Cloud. It is you defining a network of your own within Amazon. You can think of a VPC like an apartment where your furniture and items are analogous to databases and instances. The walls of your apartment isolate and protect your things from being accessible to other tenants of the apartment complex.

  • Subnets would then be analogous to the different rooms in your apartment. They are containers within your VPC that segment off a slice of the CIDR block you define in your VPC. Subnets allow you to give different access rules and place resources in different containers where those rules should apply. You wouldn't have a big open window in your bathroom on the shower wall so people can see you naked, much like you wouldn't put a database with secretive information in a public subnet allowing any and all network traffic. You might put that database in a private subnet (i.e. a locked closet).

Last updated: 2022-05-14