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