How a firewall decides what gets in

How a firewall decides what gets in

A firewall is a list, read from the top.

Every packet carries a sender and a port. The firewall checks it against rule 1, then rule 2, then rule 3, and the first rule that matches decides. Nothing below it is read.

That's why order matters: the office gets in on port 22 because rule 2 sits above the rule that drops everyone else. And anything that matches no rule at all is dropped by the default at the bottom.

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