ANSWER : SUBNET MASK IS 255.255.255.224
CAUSE 11 BIT REQUIRED FOR SUBNET ID TO CREATE - 2000 SUBNET
AND THERE IS 15 USABLE HOST IP ADDRESS +2 IP ADDRESS REQUIRE IN SUBNET (FOR SUBNET ID AND DBA)
SO FOR TOTAL 17 IP ADDRESS ,WE REQUIRE 5 BIT HID .
TOTAL 16 BIT = 11+5 ARE USED
FROM THAT 11 BIT IS SID AND ALREADY 16 BIT NID TOTAL 27 BIT ARE USEFUL FOR MASK