SELECT   COUNT(distinct ip) AS [Number of visitors]
FROM Table1
where CONVERT(VARCHAR(10),(Date_server), 105) =   '3-10-2011'

SELECT ROW_NUMBER() OVER (PARTITION BY ip ORDER BY Date_server ASC) AS Rn, * INTO #Temp
FROM Table1
WHERE Date_server> = '2011-10-03'
AND Date_server < '2011-10-04'

;With Visitors
AS
(
SELECT Rn,Date_server , IP,CAST(IP AS varchar(100)) AS DistVisit
FROM #Temp
WHERE Rn=1
UNION ALL
SELECT t.Rn,t.Date_server,t.IP,CAST(CASE WHEN DATEDIFF(mi,v.Date_server,t.Date_server) > 30 THEN t.IP + CAST(t.Rn AS varchar(10)) ELSE t.IP END AS varchar(100))
FROM Visitors v
INNER JOIN #Temp t
ON t.IP = v.IP
AND t.Rn = v.Rn + 1
)

SELECT COUNT(DISTINCT DistVisit)
FROM Visitors

DROP TABLE #Temp