Causes for network related errors in SQL Server

Following are the main reasons for network related errors in SQL Server.


Firewall does not allow connections to SQL Server Service.
Solution: Add exception to Firewall to allow connections to SQL Server Service.


Firewall does not allow connections to SQL Browser Service.
Solution:  Add exception to Firewall to allow connections to SQL Browser Service.


Server/Instance name is wrong.
Solution: Check for the correct name of the Server/Instance and try to connect again.


TCP/IP is disabled on the Server Machine.
Solution: Enable TCP/IP
Steps to enable TCP IP On SQL Server
Open SQL Server Configuration Manager
- All programs ->(Microsoft SQL Server 2008 R2 (or) Microsoft SQL Server 2008 (or) Microsoft SQL Server 2005 ) -> SQL Server Configuration Manager
- Expand SQL Server Network configuration
- Click on protocols for MSSQLSERVER (If its an instance click on the respective instance name)
- RightClick on TCP/IP and click on enable


SQL Server Browser Service is not running on the Server Machine.
Solution: Start the SQL Server Browser service on the Server Machine.
Steps to start the SQL Server Browser Service:
- Run -> Services.msc
- RightClick on SQL Server Browser
- Click on Start


Remote connections are disabled on SQL Server
Solution: Allow remote connections on sql server
Steps:
- Connect to SQL Server on SSMS
- RightClick on Server
- Click On Properties
- In the Server Properties Popup Select Connections
- Check the Allow remote connections to Server and click on OK


SQL Server Service is not running on the Server Machine.
Solution: Start the SQL Server service on the Server Machine.
Steps to start the SQL Server Service:
- Run -> Services.msc
- RightClick on SQL Server (MSSQLSERVER)
- Click on Start


SQL Server instance Service is not running on the Server Machine.
Solution: Start the SQL Server instance service on the Server Machine.
Steps to start the SQL Server instance Service:
- Run -> Services.msc
- RightClick on SQL Server (instancename)
- Click on Start


Invalid Credentials
Solution: Fetch the latest creadentials to login into the server or ask the administrator to give your credentials necessary oermissions on the server.


Time out errors due to network latency
Solution:  Increase Remote Query Time out
Steps:
- Connect to SQL Server on SSMS
- RightClick on Server
- Click On Properties
- In the Server Properties Popup Select Connections
- Increase the Remote query timeout value and click on OK


Ref: http://sqlserverlearner.com/2012/causes-for-network-related-errors-in-sql-server