As far as my knowledge in oracle,

1. When the Oracle client wants to Eshtablis connection with any of the oracle servers, net8 configuration has to be done .And the details about the server will be entered in the Tnsnames File.Every time you want to connect to the server the tnsnames files will be read and the server details will be used to connect with the server.


2. The listener process will be in the Oracle Server which will be constantly listening on the port, and if any client wishes to connect to the server hands off the request to the listener and the listener takes care of the rest in authenticating and connecting.( also it will handle to request to the dispatcher in the case of Shared server mode)