Question: I want to find missing numbers in a range?

Answer: C1 and C2 contains the range boundaries, A2:A6 (List) contains the list.

Formula in B5:B15:

=SMALL(IF(ISERROR(MATCH(ROW(INDIRECT(C1&":"&C2));List;0));ROW(INDIRECT(C1&":"&C2));"");ROW()-ROW(missing_list_start)+1) + CTRL + SHIFT + ENTER

Excel 2007 users can remove any #NUM errors with this formula:

=IFERROR(SMALL(IF(ISERROR(MATCH(ROW(INDIRECT(C1&":"&C2));List;0));ROW(INDIRECT(C1&":"&C2));"");ROW()-ROW(missing_list_start)+1);"") + CTRL + SHIFT + ENTER

Named ranges
List (A2:A6)
missing_list_start (B5)

What is named ranges?

How to customize the formula to your excel spreadsheet
Change the named ranges.

Download excel sample file for this tutorial.
missing-values-in-a-column.xlsx
(Excel 2007 Workbook *.xlsx)

Functions in this article:

MATCH(lookup_value;lookup_array; [match_type]
Returns the relative position of an item in an array that matches a specified value

INDEX(array,row_num,[column_num])
Returns a value or reference of the cell at the intersection of a particular row and column, in a given range

IF(logical_test;[value_if:true];[value_if_false])
Checks whether a condition is met, and returns one value if TRUE, and another value if FALSE

INDIRECT(ref_text,[a1])
Returns the reference specified by a text string

ROW(reference) returns the rownumber of a reference

SMALL(array,k) Returns the k-th smallest row number in this data set.