So I'm having this issue after installing MS SQL Server 2008 R2 Enterprise edition, I'm getting this message error:

Unable to connect to WIN7 \ MSSQLSERVER

A network-related error or instance-specific 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: SQL Network Interfaces , error : 25 - Connection string is not valid ) (Microsoft SQL Server, Error: 87)

NB: Before installing this version i did remove an existing MSSQL 2008 R2 EXPRESS, but after launching "SQL Server Management Studio" i strangely got for the first time the following instance of the express edition version: WIN7 \ SQLEXPRESS

My configs are: Windows 7 Enterprise Edition SQL SERVER 2008 R2 Enterprise

the SQL connection session: - DB Engine - WIN7 \ MSSQLSERVER - Windows Authentication

Please HELP !!

  • Wow, I couldn’t find the instance that I’ve created in my first install in the find server browse ! Nov 12, 2014 at 8:33

2 Answers 2


For a default instance (MSSQLSERVER), you only specify the computer name when connecting; the instance name (which is actually a ficticious placeholder) is implied.

Also, if you're connecting from another machine now, you may need to configure the Windows Firewall to allow connections through. The error message you get will be the same as the one you're seeing now. See my video here for more on that.

  • Thank you, I’ve created a second instance, it did work, but when a saw you post, I’ve tested to connect with my PC hostname only and it did work, indeed for the default instance we only specify the computer name, thanks again :) Nov 12, 2014 at 9:13
  • @Strassenger: You're welcome.
    – Jon Seigel
    Nov 12, 2014 at 14:57

Open SQL Server Configuration Manager. That will likely show you the answer to your problem. In SSCM you should be able to see what database instances are installed, what their instance names are, what ports they run on, and whether the services are up.

There could be a number of different reasons you're getting this such as the server is not started or you didn't install the server using the instance name you think you did.

  • Sorry but I try to browse to see all my instance, it surprise me that I didn’t find any one, but they exists in the : SQL Server configuration manager and in services of windows !!! Nov 12, 2014 at 8:28

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