How to Create Dynamic Sub-domain with ASP.NET?

The first step is to place the subdomain name in the DNS Host Server. To do that you need to manipulate the dns files. For example if you use BIND as DNS server you go and open the text file that keep your DNS configuration eg: "c:\program files\dns\var\mysite.com" and there you add a line as

subdomain.mysite.com.   IN  A   111.222.333.444

Also you change the ID of the file to give a message to BIND to update the subdomains.

Second step is to redirect the new subdomain to the correct directory. You do that on the protected void Application_BeginRequest(Object sender, EventArgs e) on Global.asax using rewritepath

protected void Application_BeginRequest(Object sender, EventArgs e)
{
    if (HttpContext.Current.Request.Url.Host.StartsWith("subdomain."))
    {
        // here you need to find where to redirect him by reading the url
        // and find the correct file.
        HttpContext.Current.RewritePath("/subdomain/" + Request.Path, false);
    }


    // .... rest code   
}

Its not so easy, not so hard... maybe there are some more minor issues like permissions to write to dns. Also you need to know dns, read the manual about.