Host Headers are used on the Windows platform to enable you to run multiple sites by using a subdomain of your choice. For example, if your website is http://mydomain.com, then you can setup numerous custom subdomains using Windows, such as http://mycompany.mydomain.com. Note: these subdomains are completely custom and are not purchased via your domain company. You will also need to have setup your IIS server as a DNS Server for your specific domain.