code: function splitVpc(cidrBlock) {
    return CIDR.fromString(cidrBlock)
      .split()
      .map(cidr => cidr.split())
      .reduce((all, halves) => all.concat(...halves))
      .map(cidr => cidr.split())
      .reduce((all, halves) => all.concat(...halves))
      .map(cidr => cidr.split())
      .reduce((all, halves) => all.concat(...halves));
  }
docs: Split a /16 CIDR block into /20 CIDR blocks.
  
  @param {String} cidrBlock VPC CIDR block
  @return {Array}