1、tnsnames.ora文件名被修改
TNS-03505: Failed to
resolve name
2、IP或hostname无法ping通
TNS-12543: TNS:destination host unreachable
3、主机名无法解析（涉及hosts文件或DNS）
TNSPING和SQLPLUS报错均为： ORA-12545: Connect failed because target host or object does not exist
4、端口未打开
ORA-12541: TNS:no listener
5、防火墙未开启端口或tns文件与listener中端口不一致
TNSPING报错为：TNS-12535: TNS:operation timed out
SQLPLUS报错为：ORA-12170: TNS:Connect timeout occurred
6、service_name指定服务名与监听中不符
TNSPING可以通。
SQLPLUS连接报错：ORA-12514: TNS:listener does not currently know of service requested in connect descriptor
7、监听未开启
ORA-12541: TNS:no listener
8、连接RAC报错
ORA-12545:Connect failed because target host or object does not exist
解决方法是：修改RAC的local_listener参数，将参数值中HOST=的值改为当前节点的VIP或者scanip--注意是IP而不是hostname，客户端可以通过SCANIP或者VIP都可以连接到RAC数据库库
详见：http://blog.csdn.net/haibusuanyun/article/details/17737553
9、共享模式连到静态监听
ORA-12523: TNS:listener could not find instance appropriate for the client
参考文档：
http://blog.csdn.net/haibusuanyun/article/details/14517211
