set asp.net custom 404 page in web.config

<configuration>
   <system.webServer>
      <httpErrors errorMode="Custom" defaultResponseMode="File" >
         <remove statusCode="404" />
         <remove statusCode="500" />
         <error statusCode="404" path="/404" responseMode="Redirect" />
         <error statusCode="500" path="500.html" />
       </httpErrors>
   </system.webServer>
</configuration>