Calling CRM over IFD

Am AD user and same user for CRM also.

but am not able to  esablish connection to my IFD CRM with network crdentilas.

here is the code

ServicePointManager.ServerCertificateValidationCallback = delegate(object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors) { return true; };
            Uri organizationUriIFD = new Uri(ServerURL);

ClientCredentials credentials = new ClientCredentials();

credentials.Windows.ClientCredential = (NetworkCredential)CredentialCache.DefaultCredentials;

IServiceConfiguration<IOrganizationService> config = ServiceConfigurationFactory.CreateConfiguration<IOrganizationService>(organizationUriIFD);

 using (Microsoft.Xrm.Sdk.Client.OrganizationServiceProxy _serviceProxy = new OrganizationServiceProxy(config, credentials))
            {
             _serviceProxy.ServiceConfiguration.CurrentServiceEndpoint.Behaviors.Add(new ProxyTypesBehavior());
           IOrganizationService _service = (IOrganizationService)_serviceProxy;
           WhoAmIResponse response = (WhoAmIResponse)_service.Execute(new WhoAmIRequest());
             UserId = response.UserId;
             return _service;
         }

Please help me.....