Pregunta sobre el TNS de ORACLE

¿Por qué no se puede resolver el nobre de mi máquina? NOTA: la BD ya esta montada y funcionando correctamente.
Pasos:
$lsnrctl start
LSNRCTL for Linux: Version 8.0.5.0.0 - Production on 06-NOV-01 15:17:19
(c) Copyright 1997 Oracle Corporation. All rights reserved.
Starting /usr/local/oracle/product/8.0.5/bin/tnslsnr: please wait...
TNSLSNR for Linux: Version 8.0.5.0.0 - Production
System parameter file is /usr/local/oracle/product/8.0.5/network/admin/listener.ora
Log messages written to /usr/local/oracle/product/8.0.5/network/log/listener.log
Listening on: (ADDRESS=(PROTOCOL=tcp)(DEV=6)(HOST=192.168.100.2)(PORT=1521))
Connecting to (ADDRESS=(PROTOCOL=TCP)(Host=leopoldo)(Port=1521))
STATUS of the LISTENER
------------------------
Alias LISTENER
Version TNSLSNR for Linux: Version 8.0.5.0.0 - Production
Start Date 06-NOV-01 15:17:24
Uptime 0 days 0 hr. 0 min. 0 sec
Trace Level off
Security OFF
SNMP OFF
Listener Parameter File /usr/local/oracle/product/8.0.5/network/admin/listener.ora
Listener Log File /usr/local/oracle/product/8.0.5/network/log/listener.log
Services Summary...
Extproc has 1 service handler(s)
Ora8 has 1 service handler(s)
The command completed successfully
Parece que todo ha ido bien
-----------------------Ooo------------------------
$sqlplus system@leopoldo
SQL*Plus: Release 8.0.5.0.0 - Production on Tue Nov 6 15:17:51 2001
(c) Copyright 1998 Oracle Corporation. All rights reserved.
ERROR:
ORA-12154: TNS:could not resolve service name
Enter user-name:
¿Qué esta pasando?