how can a Visual Basic Web App project read the data after the '?' in the URL to the site?

The URL given out to our web site will have some ID info tacked on at the end of it.
eg.
www.ourSite.com/default.aspx?id123
How can my default.aspx.vb read the 'id123' from the URl that the user used to get to www.ourSite.com/default.aspx
?