Oracle Net Services provides methods for understanding and resolving network problems through the use of log and trace files. These files keep track of the interaction between network components as errors occur. Evaluating this information will help you to diagnose and troubleshoot even the most complex network problems.

see detail instruction how to troubleshoot Oracle NET issues.

http://download.oracle.com/docs/cd/B19306_01/network.102/b14212/troublestng.htm#i435776