Faced with such a task. It is a network of Windows 7 machines, the topology of the network as such is absent — all machines access the Internet via the switch. It is necessary to configure filtering web content. Before such problems are solved, the Linux gateway with a proxy server.
In this case, unfortunately, not an option. So I thought, maybe there is a client-server software under Windows to resolve this problem. The essence in what traffic to let through a single server will not work, so you need softwaree decision. For example, one machine as a proxy server, but not at the network level and at the level of the software.
Now I will search but maybe someone can advise.