cloudformation needs fn::split

gistfile1.txt

{
  "ReversePrivateDNSRecord1" : {
    "Type" : "AWS::Route53::RecordSet",
    "Properties" : {
      "HostedZoneId" : "XXXXXXXXXXX",
      "Comment" : { "Fn::Join" : [ "", ["DNS name for instance in CF stack: ", { "Ref": "AWS::StackName" }]]},
      "Name" : {
          "Fn::Join" : [ "", [
             { "Fn::Select" : [ "3", {"Fn::Split" : [".", { "Fn::GetAtt" : [ "Host1", "PrivateIp" ] } ] }]},
             ".",
             { "Fn::Select" : [ "2", {"Fn::Split" : [".", { "Fn::GetAtt" : [ "Host1", "PrivateIp" ] } ] }]},
             ".10.10.in-addr.arpa."
        ] ]
      },
      "Type" : "PTR",
      "TTL" : "900",
      "ResourceRecords" : [{"Fn::Join" : [ "", [{ "Ref" : "InstanceName" }, ".MYDOMAIN.net."] ]}]
    }
  }
}