* Returns the {@linkplain #isRegistrySuffix() registry suffix} portion of the domain name, or
   * {@code null} if no registry suffix is present.
   *
   * @since 23.3
   */
  public @Nullable InternetDomainName registrySuffix() {
    return hasRegistrySuffix() ? ancestor(registrySuffixIndex()) : null;
  }

  /**
   * Indicates whether this domain name ends in a {@linkplain #isRegistrySuffix() registry suffix},