Why can't iPhone 4s connect to a specific IP address? I built a robot a while ago that I control via the iPhone (video).  

The robot connects to a WiFi hotspot and the iPhone 3Gs would connect to it via Safari and I'd control it from there.  Now that I've upgraded to iPhone 4s, Safari simply won't connect to the IP address stating "Cannot open page".

I can't figure out why.  Every other device in the house connects to the robot fine. How can I debug this?
Are you sure your new iPhone is connected to the same WiFi access point?  

Try turning both Airplane mode and WiFi ON in Settings to make sure you are accessing the correct local network.