There are various layers of a security system. First, there is the physical layer. This refers to the actual hardware. When a system is firewalled physically, there is no connection with outside networks. While this method can be very effective, it is limiting and not as flexible or fast as other methods. The second kind of firewall layer is the application type. This is at the software level where bits of code determine what kind of information is passing through. This filtering is done via a process known as packet filtering. For more details:  firewall engineer
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