I want to prevent certain applications from accessing the internet. When I was using windows XP I had an application (i think it was Zone alarm) which always notified me when an application tried to access the internet. I wonder if there is an application in Ubuntu which would do the same task. If it is possible, please suggest a gui application.

the problem is, my internet provider allows me to connect to a single host at a time, that means no two applications can use internet at the same time. When i checked system monitor it showing me that internet is working fine still my browser cant browse.So i want to kill the other application using the internet to browse.