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# `Fn::Cidr`
<a name="intrinsic-function-reference-cidr"></a>

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
<a name="intrinsic-function-reference-cidr-declaration"></a>

### JSON
<a name="intrinsic-function-reference-cidr-syntax.json"></a>

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

### YAML
<a name="intrinsic-function-reference-cidr-syntax.yaml"></a>

Syntax for the full function name:

```
Fn::Cidr: 
  - ipBlock
  - count
  - cidrBits
```

Syntax for the short form:

```
!Cidr [ ipBlock, count, cidrBits ]
```

## Parameters
<a name="intrinsic-function-reference-cidr-parameters"></a>

ipBlock  <a name="ipBlock"></a>
The user-specified CIDR address block to be split into smaller CIDR blocks.

count  <a name="count"></a>
The number of CIDRs to generate. Valid range is between 1 and 256.

cidrBits  <a name="cidrBits"></a>
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".  
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
<a name="intrinsic-function-reference-cidr-return-values"></a>

An array of CIDR address blocks.

## Example
<a name="intrinsic-function-reference-cidr-examples"></a>

### Basic usage
<a name="intrinsic-function-reference-cidr-example1"></a>

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

#### JSON
<a name="intrinsic-function-reference-cidr-example1.json"></a>

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

#### YAML
<a name="intrinsic-function-reference-cidr-example1.yaml"></a>

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

### Creating an IPv6 enabled VPC
<a name="intrinsic-function-reference-cidr-example2"></a>

This example template creates an IPv6 enabled subnet.

#### JSON
<a name="intrinsic-function-reference-cidr-example2.json"></a>

```
{
    "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
<a name="intrinsic-function-reference-cidr-example2.yaml"></a>

```
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
<a name="intrinsic-function-reference-cidr-functions"></a>

You can use the following functions in a `Fn::Cidr` function:
+ [`Fn::Select`](intrinsic-function-reference-select.md)
+ [`Ref`](intrinsic-function-reference-ref.md)