I found the solution:
First : IIS authentication should be Windows Authentication
Second : a Domain user should be inserted in web.config in this format :
<configuration><appsettings>
<add key="LDAPsvcAcct" value="domain\username" />
<add key="LDAPsvcPass" value="user_password" />
</appsettings></configuration>
Third : then you can use this code:
Dim Username As String = ""
Dim identityName = User.Identity.Name
Using HostingEnvironment.Impersonate()
Using context = New PrincipalContext(ContextType.Domain, "Domain.com", Nothing, ContextOptions.Negotiate Or ContextOptions.SecureSocketLayer)
Using userPrincipal__1 = UserPrincipal.FindByIdentity(context, IdentityType.SamAccountName, identityName)
EmpID = userPrincipal__1.EmployeeId
Session("EmpID") = userPrincipal__1.EmployeeId
Username = userPrincipal__1.DisplayName
End Using
End Using
End Using