[Oracle] ORA-12547: TNS:lost contact

[mon@topaz source]$ export ORACLE_BASE=/u01/oracle
[mon@topaz source]$ export ORACLE_HOME=$ORACLE_BASE/product/11.2.0/dbhome_1
[mon@topaz source]$ export ORACLE_SID=xxxxx
[mon@topaz source]$ sqlplus / as sysdba
-bash: sqlplus: command not found
[mon@topaz source]$ $ORACLE_HOME/bin/sqlplus / as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Sun Feb 28 14:07:48 2016

Copyright (c) 1982, 2009, Oracle.  All rights reserved.

ERROR:
ORA-12547: TNS:lost contact

[oracle@topaz bin]$ ls -l oracle
-rwxrwxr-x 1 oracle oinstall 210824714 May  4  2014 oracle
[oracle@topaz bin]$ 
[oracle@topaz bin]$ chmod 6751 oracle
[oracle@topaz bin]$ ls -l oracle
-rwsr-s--x 1 oracle oinstall 210824714 May  4  2014 oracle

[mon@topaz source]$ export ORACLE_BASE=/u01/oracle
[mon@topaz source]$ export ORACLE_HOME=$ORACLE_BASE/product/11.2.0/dbhome_1
[mon@topaz source]$ export ORACLE_SID=xxxxx
[mon@topaz source]$ $ORACLE_HOME/bin/sqlplus /nolog

SQL*Plus: Release 11.2.0.1.0 Production on Sun Feb 28 14:10:06 2016

Copyright (c) 1982, 2009, Oracle.  All rights reserved.

SQL> conn / as sysdba
Connected.
SQL> exit