Define URL structure

Hello, I have one wordpress installation under the root of my website. Right now for all my posts the URLs are http://www.site.com/post-slug/.

Is there a way, when I’m posting, to set that the URL will become with a subdomain instead? I mean http://articles.site.com/post-slug/.

So, if I’m posting a simple short entry, it will apper under the root of the site. If I’m posting an entire article, it will appear under the subdomain ‘articles.site.com’.