Firewall
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def
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a device (or software) that controls network traffic based on a set of rules
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rules
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evaluated in a logical, top-to-bottom path — general rules toward the bottom, more specific rules toward the top; see Access Control List (ACL) — filters on source/destination IP, port number, time of day, application, etc.
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implicit deny
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most firewalls include an implicit "deny everything else" rule at the very bottom, even if you didn't add one explicitly
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