using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
 
public class IPBlockModule : IHttpModule
{
       public IPBlockModule()
       {
              // TODO: Add constructor logic here
       }
    public void Dispose()
    {
        //Dispose
    }
 
    public void Init(HttpApplication context)
    {
        context.BeginRequest += new EventHandler(Application_BeginRequest);
    }
 
    private void Application_BeginRequest(object source, EventArgs e)
    {
        HttpContext context = ((HttpApplication)source).Context;
        string ipAddress = context.Request.UserHostAddress;
        if (IsBlockedIpAddress(ipAddress))
        {
            context.Response.StatusCode = 403;
            context.Response.Write("Forbidden : The server understood the request, but It is refusing to fulfill it.");
        }
    }
 
    private bool IsBlockedIpAddress(string ipAddress)
    {
        //Here I have stored Ip addresses in String[]. you can also Store in database.
        string[] IPs = {            "117.196.35.121",
                                    "117.196.35.122",
                                    "117.196.35.123",
                                    "117.196.35.124",
                                    "127.0.0.1"
                       };
 
        foreach(string IP in IPs)
        {
            if(IP == ipAddress)
                return true;
        }
        return false;
   
    }
    
}

 <configuration>
       <system.web>
              <compilation debug="true" targetFramework="4.0"/>
              <httpModules>
                     <add name="IPBlockModule" type="IPBlockModule"/>
              </httpModules>
       </system.web>
</configuration>