ORA-12537:Tns connection Closed in Toad

Error : 

ORA-12537 TNS:connection closed

 Cause:

* In the Sqlnet.ora file the parameter TCP.VALIDNODE_CHECKING is enabled and TCP.INVITEDNODES is set to some specific IP's of the Client Machine.

Solution:

Comment the following line in sqlnet.ora and  try again or restart the listener

          tcp.validnode_checking = yes