different permalinks for “pages” and “posts”

Hello,

i need help with .htaccess modrewrite.

I’m using permalinks in WP like this:

/%post_id%/%postname%.html

So it’s like

http://www.sitename.com/123/name-of-post.html

But if it is a page, not a post, it appears:

http://www.sitename.com/name-of-post

How can i change this?
I mean, i want the pages URL like

http://www.sitename.com/pages/name-of-post.html

Any idea?

Thanks in advance.