1. <connectionStrings>
2. <add name="yourconnectinstringName" connectionString="Data Source= DatabaseServerName; Integrated Security=true;Initial Catalog= YourDatabaseName; uid=YourUserName; Password=yourpassword; " providerName="System. Data. SqlClient" />
3. </connectionStrings>

1. <connectionStrings>  
2.     <add name="yourconnectinstringName" connectionString="Data Source= DatabaseServerName; Integrated Security=true;Initial Catalog= YourDatabaseName; uid=YourUserName; Password=yourpassword; " providerName="System.Data.SqlClient" />   
3. </connectionStrings>  

<connectionStrings>       <add name="yourconnectinstringName" connectionString="Data Source= DatabaseServerName; Integrated Security=true;Initial Catalog= YourDatabaseName; uid=YourUserName; Password=yourpassword; " providerName="System.Data.SqlClient" />    </connectionStrings> 

'Connectionstring' is the variable to use the connection string in any class of the .net.

ip_address_of_server : represents the ip of the server

databasename : database name

username : name of the database user

password : password of the database associated with the user

Use belo code in web.config file 

<appSettings>
<add key="connectionstring" value="server=ip_address_of_server;database=databasename;uid=username;pwd=password"/>
</appSettings>

namespace DemoApplication
{  
	public partial class Demo  System.Web.UI.Page  
    {  
	  protected void Page_Load(object sender, EventArgs e)  
	  {  
		string connetionString;
		SqlConnection cnn;
            
		connetionString = @"Data Source=WIN-50GP30FGO75;Initial Catalog=Demodb ;User ID=sa;Password=demol23";
			
		cnn = new SqlConnection(connetionString);
			
		cnn.Open();  
			
		Response.Write("Connection MAde");    
		conn.Close();  
			
	  }
	}
}

After opening the web.config file in application, add sample db connection in connectionStrings section like this:

Declaring connectionStrings in web.config file:

Now, write the code to get the connection string from web.config file in our codebehind file. Add the following namespace in codebehind file.

using System.Configuration;

This namespace is used to get configuration section details from web.config file.

C# code