SQL & PL/SQL :: How To Grep Client IP Address When LOGON

How to grep client's IP address when LOGON to database? I write a trigger , but when compile , it show Error , Here is code :

CREATE OR REPLACE TRIGGER On_Logon
AFTER LOGON ON The_user.Schema
DECLARE
v_addr VARCHAR2(11);
BEGIN
IF (ora_sysevent = 'LOGON') THEN
v_addr := ora_client_ip_address;
raise_application_error( '-20001', 'user IP: '||v_addr);
END IF;
END;
[code]....