we have built two new application servers A and B. while user is trying to access the database server C from A and B he is facing error "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 SQL Server is configured to allow remote connections" There are other application servers connected to C and working fine. Only probelm with New servers A and B.
Can you ping the SQL server? Does nslookup resolve the host name of the SQL server? Is SQL server running on the default port or is it a named instance? The rest of my suggestions are only relevent if it NOT running on the default port. The most likely problem now is that something is preventing the client from determining the port of the SQL instance. This is done using the SQL browser service, so as others have suggested, check the browser service is running on the SQL server. As others can connect I expect it is running. My next suggestion is to turn off the windows firewall on one of the application servers. I have had problems where the windows firewall prevents the reply from the SQL Browser. If the connection now works then you can either leave it turned off or add an appropriate rule to allow the SQL Browser traffic. SQL Browser operates on UPD port 1434, so either allow that port or you can add an allow rule for the application that needs to connect to SQL. It is also possible that the firewall on the SQL server is blocking the browser service, but I think that is unlikely in your case becuase you said you are only having issues with two new application servers.