Fn::Cidr - AWS CloudFormation
DeclarationParametersReturn valueExampleSupported functions

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: