You need url routing of ASP.NET, and it's available since .NET 3.5 SP1.
For your case, I think it's easier to "route" instead of rewriting, and it's simpler to use.
Why? MSDN said this:
In ASP.NET routing, you define URL patterns that contain placeholders
for values that are used when you handle URL requests. At run time,
the pieces of the URL that follow the application name are parsed into
discrete values, based on a URL pattern that you have defined. For
example, in the request for
http://server/application/Products/show/beverages, the routing parser
can pass the values Products, show, and beverages to a handler for the
request. In contrast, in a request that is not managed by URL routing,
the /Products/show/beverages fragment would be interpreted as the path
of a file in the application.
You can also use the URL patterns to programmatically create URLs that
correspond to the routes. This enables you to centralize the logic for
creating hyperlinks in your ASP.NET application.
ASP.NET Routing versus URL Rewriting
ASP.NET routing differs from other URL rewriting schemes. URL
rewriting processes incoming requests by actually changing the URL
before it sends the request to the Web page. For example, an
application that uses URL rewriting might change a URL from
/Products/Widgets/ to /Products.aspx?id=4. Also, URL rewriting
typically does not have an API for creating URLs that are based on
your patterns. In URL rewriting, if you change a URL pattern, you must
manually update all hyperlinks that contain the original URL.
With ASP.NET routing, the URL is not changed when an incoming request
is handled, because routing can extract values from the URL. When you
have to create a URL, you pass parameter values into a method that
generates the URL for you. To change the URL pattern, you change it in
one location, and all the links that you create in the application
that are based on that pattern will automatically use the new pattern.
See ASP.NET Routing in MSDN Library.