ORA-12537 TNS: Connection Closed

When I was trying to establish client connection to one of our databases running on Oracle 11g, I received “ORA-12537 TNS: Connection Closed” error.
Upon investigation, I came across “TCP.VALIDNODE_CHECKING” parameter in “sqlnet.ora” file. This was set to “YES”. please set tp "NO" or   simply commented this parameter and every thing went fine. Clients were able to establish connection to the database. By setting TCP.VALIDNODE_CHECKING to “YES”, you inform Oracle database to allow/deny connections to the listed nodes. You may list the invited nodes using TCP.INVITED_NODES parameter in sqlnet.ora file or, alternatively, you may use the TCP.EXCLUDED_NODES parameter to disallow database connections from the listed nodes. Hope you find this information helpful!!!