WiFi connects to router but cannot connect to internet after upgrading to OS X Mountain Lion

I have recently upgraded my Macbook Pro (late 2011) to the new OS X Mountain Lion.

Before this upgrade, my WiFi was working perfectly fine. However, immediately after installing Mountain Lion, I no longer received internet access through WiFi.

According to System Preferences, I do have a connection to my router and I have an IPv4 address (starting with 192.168), but when I open a browser or use any application which requires internet access, it says it cannot connect. I have tried rebooting the router but the problem persists.

What can I do to fix this?

I am using the internet through Ethernet right now (which works perfectly fine).

Edit: I have already re-installed Mountain Lion. That didn't fix it.