In order to better assist troubleshooting connectivity related issues, the more specific you describe the problems, the faster we can help you out. Please take a few minutes to answer the following questions and post them along with your specific repro steps.
Check out this blog site first, it may already have your questions answered. Otherwise, you can post your questions on
 Client side:
- What is the connection string in you app or DSN? (please specify)
- If client fails to connect, what is the client error messages? (please specify)
- Is the client remote or local to the SQL server machine? [Remote | Local]
- Can you ping your server? [YES | NO ]
In cmd.exe console, type “ping -a <server_name>”.
- Can you telnet to your SQL Server? [YES | NO, please specify the error message ]
In cmd.exe console, type “telnet <server name> port, where port can be 135, 445 or sql_server_tcp_port. If your cmd.exe console turns into a complete black screen with a cursor flushing on top left corner, you are connected. Type ctrl+’[‘ to bring up telnet prompt and type “quit” <enter>.
- What is your client database provider? [SNAC | MDAC | ADO.NET1.0 | ADO.NET2.0| other (please specify] Or/And, what is your client application? [SQL Management Studio | SQL Profiler | Visual Studio | Other (please specify).
- Is your client computer in the same domain as the Server computer? (Same domain | Different domains | WorkGroup)
- What protocol the client enabled? [Shared Memory | TCPIP | Named Pipes].
- Do you have aliases configured that match the server name portion of your connection string? If so, please check if it is correct.
 Server side:
- What is the MS SQL version? [ SQL Server 2005 | SQL Sever 2005 ]
- What is the SKU of MS SQL? [Enterprise | Standard | Workgroup | Express (or MSDE) | other (please specify)].
- What is the SQL Server Protocol enabled? [Shared Memory | TCPIP | Named Pipes ]. Use SQL Server Configuration Manager to configure it and check ERRORLOG or event log to confirm.
- Does the server start successfully? [YES | NO] If not what is the error messages in the SQL server ERRORLOG?
- If SQL Server is a named instance, is the SQL browser enabled? [YES | NO]
- What is the account that the SQL Server is running under?[Local System | Network Service | Domain Account]
- Do you make firewall exception for your SQL server TCP port if you want connect remotely through TCP provider? [YES | NO | not applicable]
- Do you make firewall exception for SQL Browser UDP port 1434? [YES | NO | not applicable ]
- What is the OS version? [Windows XPSP2 | Windows 2003 | Windows 2000 | Windows 98 | others (please specify ) ].
- Do you have third party antivirus, anti-spareware software installed? [Symantec | Norton | other (please specify)].
- If you have certificate configuration issue: Please use “certutil.exe –v –store my” to dump certificate specific info and post it in your question.
1. Find SQL Server Errorlog: Default to C:Program FilesMicrosoft SQL ServerMSSQL.#MSSQLLOG
 Last but not least:
There is a wealth of information already available to help you answer your questions.
(1) SQL Server 2005 Books Online: http://msdn2.microsoft.com
(2) Microsoft Support Knowledge Base: http://support.microsoft.com
(3) SQL Protocol Team Blog: http://blogs.msdn.com/sql_protocols
(4) MSN Search: http://search.msn.com/ or use your favorite search engine.