1、tnsnames.ora文件名被修改

TNS-03505: Failed toresolve 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 targethost or object does not exist

4、端口未打开

ORA-12541: TNS:nolistener

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 notcurrently know of service requested in connect descriptor

7、监听未开启

ORA-12541: TNS:nolistener

8、连接RAC报错

ORA-12545:Connectfailed 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