Firewalls can function in multiple modes. A legacy operating mode is stateless firewalling, also known as packet filtering. This technique involves configuring the firewall with rules to allow specific data in and keep specific data out of the network. The data going in and out has no relationship to each other, as the firewall is only opening ports on both sides or restricting access based on IP addresses.
Modern firewalls offer stateful packet inspection, meaning they understand and keep track of all the flows that pass through the device. They only allow traffic to flow between specific devices, for example . . .
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