Thomas, check out ASP.NET Routing.
In a nutshell, ASP.NET Routing is a library that was introduced in the .NET Framework 3.5 SP1 that decouples the URL from a physical file. It is used heavily in ASP.NET MVC, but can also be used in WebForms applications. I have authored an article that discusses how to use ASP.NET Routing in an ASP.NET 3.5 SP1 WebForms application: Using ASP.NET Routing Without ASP.NET MVC. It includes a complete working demo you can download and try out on your computer.
From the article:
ASP.NET Routing is a library that was introduced in the .NET Framework 3.5 SP1 that decouples the URL from a physical file; it is used extensively in ASP.NET MVC web applications. With ASP.NET Routing you, the developer, define routing rules that indicate what route patterns map to what physical files. For example, you might indicate that the URL Categories/CategoryName maps to the ShowProductsByCategory.aspx ASP.NET page, passing along the CategoryName portion of the URL. The ASP.NET page could then display the products for that category. With such a mapping, users could view products for the Beverages category by visiting www.yoursite.com/Categories/Beverages rather than visiting the more verbose and less readable www.yoursite.com/ShowProductsByCategory.aspx?CategoryID=1.
While ASP.NET MVC is a great way to get started with ASP.NET Routing, the good news is that these two systems are independent of one another. It's quite possible to use ASP.NET Routing in a traditional ASP.NET Web Forms application. This article shows how to get ASP.NET Routing up and running in a Web Forms application. Read on to learn more!
To use ASP.NET Routing you'll need to be using ASP.NET 3.5 SP1 or ASP.NET 4. In fact, there were a number of enhancements to ASP.NET Routing in ASP.NET 4 to make it easier to use in a WebForms application, so if you can upgrade to ASP.NET 4 that might be helpful.
Happy Programming!