oracle@oraserv:~/product/10gR2/network/admin> cat listener.ora
# listener.ora Network Configuration File: /opt/oracle/product/10gR2/network/admin/listener.ora
# Generated by Oracle configuration tools.

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = PLSExtProc)
      (ORACLE_HOME = /opt/oracle/product/10gR2)
      (PROGRAM = extproc)
    )
    (SID_DESC =
      (SID_NAME = testdb)
      (GLOBAL_DBNAME = testdb)
      (ORACLE_HOME = /opt/oracle/product/10gR2)
    )  
    (SID_DESC =
      (SID_NAME = orcl)
      (GLOBAL_DBNAME = orcl)
      (ORACLE_HOME = /opt/oracle/product/10gR2)
    )  
  )

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1))
      (ADDRESS = (PROTOCOL = TCP)(HOST = 10.10.10.1)(PORT = 1521))
      (ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))
    )
  )