Want to modify drupal default pager functionality
I worked on it and found a result... actually Drupal core pager is user friendly enough but in case you need to alter it's functionality you need to edit variables in theme_pager() in your theme's template file.

To obtain the above functionality I altered some variables which are as follows...

$pager_first = (floor($pager_current / $quantity) * $quantity) + 1;

$pager_last = $pager_first + $quantity - 1;


if( $pager_current % $quantity == 0){
  $pager_first = $pager_current - $quantity +1;
  $pager_last = $pager_first + $quantity - 1;
}


I neglected $page_middle variable since I did not needed it...

Warning: Do this when its absolutely necessary...