Find out Missing Numbers in a sequence

Example:

We have a table ORDH, which is having a field ORDERNO which starts from 300000. currently there are around 100 thousand records exists in this table.
The ORDERNO value is generated from a sequence.
We want to find out the missing (hole in the sequence) ORDERNO from the ORDH table. 
(Ex: 300654 is missing and there are lot of other order numbers were also missing)
Is there a way to find out all the missing order numbers from this table?

And We Said....

WITH     got_next_orderno     AS
(
     SELECT     orderno
     ,     LEAD (orderno) OVER (ORDER BY orderno)     AS next_orderno
     FROM     ordh
)
SELECT     ordrno + 1          AS from_no
,     next_orderno - 1     AS to_no
FROM     got_next_orderno
WHERE     orderno     + 1     != next_orderno
;

OR you can use this code as well:

select min_a - 1 + level
      from ( select min(id) min_a
                  , max(id) max_a
               from d3
           )
    connect by level <= max_a - min_a + 1
     minus
     select id
       from d3