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AWS CloudFormation
User Guide (API Version 2010-05-15)

Fn::Cidr

The intrinsic function Fn::Cidr returns an array of CIDR address blocks. The number of CIDR blocks returned is dependent on the count parameter.

Declaration

JSON

{ "Fn::Cidr" : [ipBlock, count, cidrBits]}

YAML

Syntax for the full function name:

Fn::Cidr: - ipBlock - count - cidrBits

Syntax for the short form:

!Cidr [ ipBlock, count, cidrBits ]

Parameters

ipBlock

The user-specified CIDR address block to be split into smaller CIDR blocks.

count

The number of CIDRs to generate. Valid range is between 1 and 256.

cidrBits

The number of subnet bits for the CIDR. For example, specifying a value "8" for this parameter will create a CIDR with a mask of "/24".

Note

Subnet bits is the inverse of subnet mask. To calculate the required host bits for a given subnet bits, subtract the subnet bits from 32 for IPv4 or 128 for IPv6.

Return Value

An array of CIDR address blocks.

Example

Basic Usage

This example create 6 CIDRs with a subnet mask "/27" inside from a CIDR with a mask of "/24".

JSON

{ "Fn::Cidr" : [ "192.168.0.0/24", "6", "5"] }

YAML

!Cidr [ "192.168.0.0/24", 6, 5 ]

Creating an IPv6 enabled VPC

This example template creates an IPv6 enabled subnet.

JSON

{ "Resources" : { "ExampleVpc" : { "Type" : "AWS::EC2::VPC", "Properties" : { "CidrBlock" : "10.0.0.0/16" } }, "IPv6CidrBlock" : { "Type" : "AWS::EC2::VPCCidrBlock", "Properties" : { "AmazonProvidedIpv6CidrBlock" : true, "VpcId" : { "Ref" : "ExampleVpc" } } }, "ExampleSubnet" : { "Type" : "AWS::EC2::Subnet", "DependsOn" : "IPv6CidrBlock", "Properties" : { "AssignIpv6AddressOnCreation" : true, "CidrBlock" : { "Fn::Select" : [ 0, { "Fn::Cidr" : [{ "Fn::GetAtt" : [ "ExampleVpc", "CidrBlock" ]}, 1, 8 ]}]}, "Ipv6CidrBlock" : { "Fn::Select" : [ 0, { "Fn::Cidr" : [{ "Fn::Select" : [ 0, { "Fn::GetAtt" : [ "ExampleVpc", "Ipv6CidrBlocks" ]}]}, 1, 64 ]}]}, "VpcId" : { "Ref" : "ExampleVpc" } } } } }

YAML

Resources: ExampleVpc: Type: AWS::EC2::VPC Properties: CidrBlock: "10.0.0.0/16" IPv6CidrBlock: Type: AWS::EC2::VPCCidrBlock Properties: AmazonProvidedIpv6CidrBlock: true VpcId: !Ref ExampleVpc ExampleSubnet: Type: AWS::EC2::Subnet DependsOn: IPv6CidrBlock Properties: AssignIpv6AddressOnCreation: true CidrBlock: !Select [ 0, !Cidr [ !GetAtt ExampleVpc.CidrBlock, 1, 8 ]] Ipv6CidrBlock: !Select [ 0, !Cidr [ !Select [ 0, !GetAtt ExampleVpc.Ipv6CidrBlocks], 1, 64 ]] VpcId: !Ref ExampleVpc

Supported Functions

You can use the following functions in a Fn::Cidr function: