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 creates 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:

• Fn::Select

• Ref