Why can't I set this computed column to Persisted?

I have a computed column in SQL Server 2008 with the following formula definition:

([dbo].[fncDot2LongIP]([IPAddress]))

Where IPAddress is another column in the table
When I try to set the Persisted flag to true, I am told that the column is not Deterministic - but that makes no sense to me given the function provided below.  I made sure not to use a Float or other such data types - what is wrong here?
FUNCTION dbo.fncDot2LongIP( 
	@IP VarChar(15) 
)
RETURNS BigInt
AS
BEGIN
	DECLARE @ipA BigInt, @ipB Int, @ipC Int, @ipD Int;
	DECLARE @thisPlace Int;
	SET @thisPlace = CHARINDEX('.',@ip);
	SET @ipA = SUBSTRING(@ip,0,@thisPlace);
	SET @ip = SUBSTRING(@ip,@thisPlace + 1, LEN(@ip));
	SET @thisPlace = CHARINDEX('.',@ip);
	SET @ipB = SUBSTRING(@ip,0,@thisPlace);
	SET @ip = SUBSTRING(@ip,@thisPlace + 1, LEN(@ip));
	SET @thisPlace = CHARINDEX('.',@ip);
	SET @ipC = SUBSTRING(@ip,0,@thisPlace);
	SET @ipD = SUBSTRING(@ip,@thisPlace + 1, LEN(@ip));
   
   RETURN ( @ipA * 256*256*256 ) + ( @ipB * 256*256 ) + ( @ipC * 256 ) + @ipD
END

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ktolaAsked:
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blandyukCommented:
You have to SELECT the value from the function:

(select dbo.fncDot2LongIP('217.34.84.90') AS IPNum)
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ktolaAuthor Commented:
Thak you for your quick response.  I tried entering this as the Formula:
 (Select dbo.fncDot2LongIP(IPAddress) as IPNum)
As I get the following error:
 Subqueries are not allowed in this context. Only scalar expressions are allowed.
 
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blandyukCommented:
mmm, just created a test table with an IP address varchar(15) and passed it into the function as a SELECT:

select dbo.fncDot2LongIP(tblTest.IPAddress) AS IPNum from tblTest

Worked perfectly. Can you post your SQL query so we can look at that?
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ktolaAuthor Commented:
That would work without an issue - however I am using the statement as the formula in a computed column.  To test this out, simply create a table with 2 columns - IPAddress VarChar(15) and LongIP.  For LongIP, start with my base formula:
([dbo].[fncDot2LongIP]([IPAddress]))

Once the table is created, then try to insert an IPValue - this will work fine but the LongIP is virtual and you cannot persist the data.
I need to know why the funciton is considered to be non-deterministic.
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Tim HumphriesDirectorCommented:
To use a UDF as a persisted column, you must use WITH SCHEMABINDING in the function definition.

Your fucntion works fine in this context if you do that. Simply place WITH SCHEMABINDING after the RETURNS clause when creating the function.
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ktolaAuthor Commented:
That was it - thanks!  I thought the WITH SCHEMABINDING was just for table functions and fields like XML.  I did not realize you could use it for scalar returns as well.
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Microsoft SQL Server 2008

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