A WebHost builder is an object , which is responsible for setting up the web server environment.These settings are completely customizable by writing our own builder. By default Web host builder is using Kestrel web server, which is a cross platform web server supported by .NET Core. This Webserver will listen all the http request connections. Also This builder set ups IIS integration. If your application are running under IIS , it is the integration that allows IIS to pass through Windows Credentials to the Kestrel server.