[root@database ~]# cat /u01/app/oracle/product/10.2.0.1/network/admin/listener.ora
# listener.ora Network Configuration File: /u01/app/oracle/product/10.2.0.1/network/admin/listener.ora
# Generated by Oracle configuration tools.

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = PLSExtProc)
      (ORACLE_HOME = /u01/app/oracle/product/10.2.0.1)
      (PROGRAM = extproc)
    )
  )

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1))
      (ADDRESS = (PROTOCOL = TCP)(HOST = database.example.com)(PORT = 1521))
    )
  )		
		
		

		
oradb10g =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = db1.domain.com)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SID = oradb10g)
    )
  )

oradb =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = db2.domain.com)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SID = oradb)
    )
  )	
		
		

$ sqlplus user@oradb
		

ORCL =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.0.5)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = orcl.example.com)
    )
  )