how query string in handler gets value?

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C#

ASP.NET

.NET

C#4.0

.NET4

there is  one handler
<%@ WebHandler Language="C#" Class="Handler" %>
using System;
using System.Web;
using System.IO;
using System.Data.SqlClient;

public class Handler : IHttpHandler 
{
   public string GetConnectionString()
    {
        return System.Configuration.ConfigurationManager.ConnectionStrings["MyConsString"].ConnectionString;
    }
    public void ProcessRequest (HttpContext context)
    {
        string id = context.Request.QueryString["id"]; 
        if (id != null)
        {    MemoryStream memoryStream = new MemoryStream();
            SqlConnection connection = new SqlConnection(GetConnectionString());
            string sql = "SELECT * FROM TblImages WHERE Id = @id";
            SqlCommand cmd = new SqlCommand(sql, connection);
            cmd.Parameters.AddWithValue("@id", id);
            connection.Open();
            SqlDataReader reader = cmd.ExecuteReader();
            reader.Read();
            //Get Image Data
            byte[] file = (byte[])reader["Image"];
            reader.Close();
            connection.Close();
            memoryStream.Write(file, 0, file.Length);
            context.Response.Buffer = true;
            context.Response.BinaryWrite(file);
            memoryStream.Dispose();
        }}
     public bool IsReusable {
        get {
            return false;    }   }}

and there is form with dropdownlist with values and there is event

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
          Image1.ImageUrl = "Handler.ashx?id=" + DropDownList1.SelectedItem.Value;                       GetImageInfo(DropDownList1.SelectedItem.Value);
       }

application is running with no error but i have one question that howcome in "id"(in handler code)value gets 1,2 or 3 (as per dropdownlist items) as qhen i run the url on browswer have only http://localhost:60686/server_controls/2.%20image_retrieving_database.aspx" then how it gets value fetched in context.Request.QueryString["id"];
pls tell regards