When clients want to connect to an Instance, they need to know where
the connectionrequest should be made, and how it should be made. The TNSNAMES.ORA file is a text file,located on each client machine, which gives the client these two crucial pieces of information.
Strictly speaking, clients don’t connect directly to an Instance at all. Instead, theyconnect to a Listener process, which knows where to establish the connection to the Instance on the client’s behalf. Therefore, a large part of the “where” information in TNSNAMES.ORA is related to identifying the machine on which the Listener is running, and the port it is listening on. There is also an element which specifies which particular Instance the client wishes to connect to, since a single Listener process can be listening on behalf of many Instances.
The “how” information in the TNSNAMES.ORA file relates mainly to the networking protocol by which the client wishes to connect to the Instance (should it be TCP/IP,SPX/IPX and so on). There is also a part which informs the Listener whether the client isseeking a dedicated or shared server connection.
http://www.scribd.com/doc/58113083/Oracle-what-is-tnsnames
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