CREATE TABLE blocks (
	startIpNum NUMERIC( 18 ) NOT NULL,
	endIpNum NUMERIC( 18 ) NOT NULL,
	lockID INTEGER REFERENCES tablename(idx_serial_id) ON DELETE CASCADE ON UPDATE CASCADE
);
CREATE INDEX idx_startIpNum ON blocks(startIpNum);
CREATE INDEX idx_endIpNum ON blocks(endIpNum);

CREATE OR REPLACE FUNCTION get_info(NUMERIC) RETURNS VARCHAR AS '
DECLARE
	_ip_num ALIAS FOR $1;
	_res INTEGER;

BEGIN
	SELECT idx_serial_id INTO _res FROM locks WHERE (startipnum <= _ip_num) AND (endipnum >= _ip_num) LIMIT 1;
	IF NOT FOUND THEN
		RAISE NOTICE ''Cant find such ip - %'',_ip_num;
		RETURN NULL;
	END IF;
	
           RETURN ...
END;
' LANGUAGE plpgsql;

SELECT idx_serial_id INTO _res FROM locks WHERE (startipnum <= _ip_num) AND (endipnum >= 
_ip_num) LIMIT  1 ;

SELECT lockid INTO _res FROM blocks WHERE (startipnum <= _ip_num) AND (endipnum >= _ip_num) ORDER BY startipnum DESC LIMIT  1 ;

explain analyze select * from blocks where startipnum<= 144666  and endipnum <=  144666  order by startipnum desc limit  1 ;

Limit  (cost= 0 . 00 .. 6 . 98  rows= 1  width= 94 ) (actual time= 29 . 398 .. 29 . 398  rows= 0  loops= 1 )
   ->  Index Scan Backward using idx_blocks on blocks  (cost= 0 . 00 .. 6 . 98  rows= 1  width= 94 ) (actual time= 29 . 392 .. 29 . 392  rows= 0  loops= 1 )
         Index Cond: ((startipnum <=  144666 ::numeric) AND (endipnum <=  144666 ::numeric))
 Total runtime:  29 . 485  ms