CREATE FUNCTION dbo.tf_LocationByIP

(
@IP varchar(15)
)

RETURNS TABLE
AS
RETURN

(
SELECT 	dbo.Locations.Locations_ID, dbo.Locations.Locations_Name, dbo.IPs.IPRange_Start, dbo.IPs.IPRange_End
FROM 	dbo.IPs INNER JOIN dbo.Locations ON
	dbo.IPRanges.Locations_ID = dbo.Locations.Locations_ID
WHERE 	CAST(PARSENAME(@IP, 2) AS INT) BETWEEN 
	CAST(PARSENAME(dbo.IPs.IPRange_Start, 2) AS INT) AND CAST(PARSENAME(dbo.IPs.IPRange_End, 2) AS INT)
)

-----------------------------------
Explanation:
@IP is the variable being passed from the workstation.
The SELECT and FROM statement tells SQL what to pull and from where.  The Locations and IPs table are joined on a common column, Locations_ID.
The WHERE clause:
	Pulls the third octet from the IP address variable and cast it as an integer (it was a number sent as a varchar).
	Pulls the third octet from the IP addresses located in the Start and End IP ranges coumns and cast it as an integer.
	Finds the data where the parsed variable IP is located between the parsed Start and End IP range octets.