First you put a packetfilter between the directly connected networks (network 1 -- packetfilter -- perimeter network -- packetfilter -- network 2). The packetfilters only allow traffic from the directly attached networks. A connection from one network to the not directly attached packetfilter or the other network is strictly forbidden.
The perimeter network is also known as DMZ (Demilitarized Zone). In it there are switching computers for all protocols/services who should work from one net to the other net. Such a switching computer is also known as Proxy, because it works pro procurationem/by proxy (like a secretary). Proxies work on application level, means they understand the communi
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