GetSubDomain

private static string GetSubDomain(Uri url)
{
if (url.HostNameType == UriHostNameType.Dns)
{
string host = url.Host;
if (host.Split('.').Length > 2)
{
int lastIndex = host.LastIndexOf(".");
int index = host.LastIndexOf(".", lastIndex - 1);
return host.Substring(0, index);
}
}

return null;
}

From: http://www.webpronews.com/expertarticles/2006/11/30/retrieve-subdomain-from-a-url-in-c