for (int i = 0; i < ipString.length(); i++) {
      char c = ipString.charAt(i);
      if (c == '.') {
        hasDot = true;
      } else if (c == ':') {
        if (hasDot) {
          return null; // Colons must not appear after dots.
        }
        hasColon = true;
      } else if (c == '%') {
        percentIndex = i;
        break; // everything after a '%' is ignored (it's a Scope ID): http://superuser.com/a/99753