VPC and Subnets: AWS Networking Services

VPC (Virtual Private Cloud) and Subnets are very important concepts under AWS Networking Services. All you AWS resources are defined in VPC. Below are some basic points about VPC and Subnets:


1. It is a logically isolated virtual network (sub-cloud) for you in AWS cloud. All your AWS resources are defined in a particular VPC.

2. You can select your own IP addresses, subnets, NACL (Network Access Control List), route tables and network gateways.

3. Whenever you create a VPC, you must define IP range for that VPC. IPv4 and IPv6 CIDR are supported. IPv4 CIDR must be from 16 to 28. IPv6 CIDR is of fixed size: 56. You cannot choose any IP range in IPv6.

4. One VPC can have multiple Subnets, NACL and Route Tables.

5. Internet Gateway: VPC is composed of subnets. Subnets are private by default. To make any subnet public, Internet Gateway should be associated with that VPC. One VPC cannot have more than one Internet Gateway.

6. VPC Peering: Connect two or more of your VPC with each other or with VPC of another AWS account. All VPC must be in same region. Example: You can enable VPC Peering between DEV VPC and UAT VPC and PROD VPC and Disaster Recovery VPC. There can be only one to one connection between VPC and Transitive Peering is not possible.

7. Whenever we create an account, a default VPC is created.

8. Default Route Table, NACL and Security Group: Whenever we create a VPC, by default one Route Table, NACL and Security Group gets created. If you don’t associate your subnets to any Route Table and NACL, this default Route Table and NACL gets associated with those subnets by default. If you don’t associate your instances to any Security Group, default Security Group is associated with each instance.

9. Flow Logs: You can associate flow logs with VPC. It captures information about the IP traffic going to and from network interfaces in your VPC. You should have IAM role for flow logs and log group in CloudWatch to enable flow logs.


1. Sub-network inside a VPC. It contains sub-range of IP Addresses in a VPC.

2. A Subnet must be associated with an AZ. It cannot spread across multiple AZ.

3. Subnet can be private and public. Keep your databases in private subnet and webservers in public subnet.

4. An instance always belongs to a subnet. You cannot have an instance in a VPC which does not belong to any subnet.

5. NACL (Network Access Control List): Optional layer of security at subnet level. Acts as firewall at subnet level (Security Group act as firewall at instance level). One subnet can only be associated with one NACL. One NACL can be associated with multiple subnets.

6. Route Table: Each subnet must be associated with a Route Table. One subnet can have only one Route Table. One Route Table can be associated with multiple subnet. Network traffic of any instance inside a subnet is dictated by the routing table attached to it.

7. While creating a subnet, you must specify VPC, CIDR (must be in between the CIDR range of the parent VPC), and Availability Zone.

8. After creating a subnet, you should associate a Route Table and NACL with it. If you don’t do this, then the default Route Table and NACL will get associated with it which was created while creating the VPC.

9. A Subnet is private by default. To make it public, 

  • Define an Internet Gateway.
  • Attach IGW to VPC. IGW should be attached to a VPC. One VPC can only be attached to one IGW. Create a Route Table and add internet route in it (direct to IGW).
  • Explicitly associate a Subnet (which you want to make public) to this Route Table. One Subnet have only one Route Table.
  • Enable Auto-assign public IPv4 address in that Subnet. You can also do this setting while launching an instance in this subnet. 
  • Ensure Security Group and NACL are not blocking internet traffic.
  • Now any EC2 instance launched in this Subnet will be able to communicate with the internet.

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