处于同一个域中的两台Sql server 实例无法连接，报的错误信息如下：
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server) (.Net SqlClient Data Provider)
错误原因是远程Server没有打开防火墙端口1433
1433端口用于远程连接，是Sql Server默认的端口，SQL Server服务使用两个端口：TCP-1433、UDP-1434。其中1433用于供SQL Server对外提供服务，1434用于向请求者返回SQL Server使用了哪个TCP/IP端口。
Solution
step1，打开FireWall，Advanced Settings
step2，New Inbound Rule，创建一个Port， TCP 1433
step3,下面的Action，Profile采用默认值，将Name命名为Sql Server Data Engine Remote connection
step4，查看在FireWall中创建的Inbound Rule
原文：http://www.cnblogs.com/ljhdo/p/4794121.html