We do a lot of work where we need to figure out which IP's are within certain subnets. I've found that the simplest and most reliable way to do this is:
ALTER FUNCTION [dbo].[IP_To_INT ]
(
@IP CHAR(15)
)
RETURNS BIGINT
AS
BEGIN
DECLARE @IntAns BIGINT,
@block1 BIGINT,
@block2 BIGINT,
@block3 BIGINT,
@block4 BIGINT,
@base BIGINT
SELECT
@block1 = CONVERT(BIGINT, PARSENAME(@IP, 4)),
@block2 = CONVERT(BIGINT, PARSENAME(@IP, 3)),
@block3 = CONVERT(BIGINT, PARSENAME(@IP, 2)),
@block4 = CONVERT(BIGINT, PARSENAME(@IP, 1))
IF (@block1 BETWEEN 0 AND 255)
AND (@block2 BETWEEN 0 AND 255)
AND (@block3 BETWEEN 0 AND 255)
AND (@block4 BETWEEN 0 AND 255)
BEGIN
SET @base = CONVERT(BIGINT, @block1 * 16777216)
SET @IntAns = @base +
(@block2 * 65536) +
(@block3 * 256) +
(@block4)
END
ELSE
SET @IntAns = -1
RETURN @IntAns
END