I want to change the subnet mask for the default subnet of my default Amazon VPC.

The CIDR block for a default VPC is always a /16 netmask ( This provides up to 65,536 private IP addresses. You have one default subnet per Availability Zone, and the netmask for a default subnet is always /20, which provides up to 4,096 addresses per subnet. For more information, see Default Subnets.

To change the subnet mask size for your default subnet, first delete your subnet; then, create a new subnet for your VPC with the preferred subnet mask as described at Adding a Subnet to Your VPC.

Note: The first four IP addresses and the last IP address in each subnet CIDR block are not available for you to use, and they cannot be assigned to an instance. For more information, see VPC and Subnet Sizing.

After you have created the subnet, open a case with AWS Support. When you open a case, you must provide your AWS account ID, the region, and the subnet ID. AWS Support will designate the new subnet as the default subnet for your default VPC.

Note: Default subnets can only be designated for a default VPC. Ensure that the new subnet you create is part of your default VPC in that region. For more information about default VPCs and subnets, see Your Default VPC and Subnets.

Amazon Virtual Private Cloud, default VPC, change default subnet mask, CIDR, Availability Zone, subnet mask size

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Published: 2016-07-11