Redirect based upon a form entry.

I'm trying to get one of two pages to load depending on a form entry.  The form consists of one text field (asking for the person's zip code),  If the zip code is found in a master table, I would like them to be directed to one page.  If it is not in the list, I would like them to be directed to another.

Any advice on how to tackle this issue?  I'm working with .asp in Dreamweaver CS4.

Thank you in advance from a novice!  -David