Your Oracle client software has a file called the TNSNAMES.ORA configuration file. This file contains the definition of the TNS alias you are using to connect to your remote database. For instance, using SQL*Plus, you can connect to a remote database similar to the following:
The "@orcl" tells SQL*Plus to connect to the database determined by the information in the TNSNAMES.ORA config file using the alias "orclno." In that alias, you specify the sid or service name of your remote database. The value for this TNA alias cannot be found in your database server's listener configuration. You'll have to talk with your DBA to find out the correct value and then update your TNS alias.
This was first published in May 2006