The problem 

I had this problem in Visual Studio 2005 and now I have it in Visual Web Developer 2008 express edition where when debugging a web application using the development web server the dynamic port number used by the server didn’t match what Visual Studio had used for the web browser. It was always wrong and normally out by two or three port numbers.

For example the cassini web development server starts and I can see in the notification area what port number it uses

However the web browser started by Visual studio looks like this

Clearly that isn’t going to work.

The Solution

I managed to track down the problem to my firewall software part of the ESET Smart Security Suite. Now, I am pretty sure that when I was using Visual Studio 2005 and had the problem that I was using either McAfee, Norton/Symantec or possibly even the Outpost firewall software. My solution won’t work if you aren’t using Eset but hopefully it will help you to look in the right area.

1. If you are using ESET Smary Security Suite then I bet you are using the Protocol Filtering (you can tell by looking in the Advanced Firewall Setup).
   You could just turn it off and ignore the rest of my blurb but you probably want to keep Protocol Filtering enabled so goto step 2.
2. In the Advanced setup you need to locate the section shown in the image below and uncheck the web development server.
   
3. Make sure you stop the web development server if it’s still running and then try debugging from Visual Studio again.

Feedback

Please let me know if this helps you or if you think the instructions can be improved. Equally if you using a different firewall and found the solution please let me know.