When CLient connects to service_name saying @xyz it should be resolved to get connect to a database to which the service_name has been registered.

You have couple of methods for names resolution:

tnasnames.ora where you describe what service_name connecting to what database on a server. When you make a connection to the client it will search tnsnames.ora nad resolve what database is supposed to connect.... in simple we call its local names resolution method.

You can have very weel name server where there are entries for all these names stored in a network. this is called names resolution using names server.

Have a read on Net8 Naming documents.

http://otn.oracle.com/docs/products/...cts.htm#439896