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
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", "Ipv6CidrBlock" ]}]}, 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.Ipv6CidrBlock], 1, 64 ]] VpcId: !Ref ExampleVpc
Supported functions
You can use the following functions in a Fn::Cidr
function: