Access Restriction

I am working with firewalls and parental controls and trying to figure out how they actually block internet access. I thought at first they somehow came between the browser and the svchost.exe process. Then I started looking through the registry and other places but I still can’t find out how the it works. I know already it does not use a proxy connection. I took the liberty of installing some software on my own computer for the experimental purposes but it restricted almost every form of editing the system.