docs: private static final boolean ipPartCheck(
              String str, int sIdx, int eIdx) 
      {
          int len = eIdx - sIdx;
          if ( len < 1 ) {
              return false;
          }
          
          switch ( len ) {
          case 1:
          case 2:
              if ( ! StringUtil.isNumeric(str, sIdx, eIdx) ) return false;
              break;
          case 3:
              char chr1 = str.charAt(sIdx);
              if ( chr1 == '1' ) {
                  if ( ! StringUtil.isNumeric(str, sIdx+1, eIdx) ) return false;
              } else if ( chr1 == '2' ) {
                  char chr2 = str.charAt(sIdx+1);
                  if ( chr2 < '0' || chr2 > '5' ) return false;
                  char chr3 = str.charAt(sIdx+2);   //the third numeric
                  if ( chr2 == '5' ) {
                      if ( chr3 < '0' || chr3 > '5' ) return false;
                  } else {
                      if ( chr3 < '0' || chr3 > '9' ) return false;
                  }
              }
              break;
          default:
              return false;
          }
          
          return true;
      }