Ensure that the client and the database server are using the same protocol. The -x option for the server selects a list of protocols for the server to use, and the CommLinks (LINKS) connection parameter does the same for the client application.
You can use these options to ensure that each application is using the same protocol.
By default, the network database server uses all available protocols. The server supports client requests on any active protocol. By default, the client only uses the shared memory protocol. The client can use all available protocols by setting the CommLinks (LINKS) connection parameter to all.