A firewall is a policy engine. Its value comes from the clarity of the policy, not the vendor logo on the box.
Stateful inspection
A stateful firewall remembers connections. If an internal host initiates an outbound connection, the return traffic is allowed without a separate inbound rule. A stateless ACL evaluates each packet independently, so return traffic needs its own rule.
Default deny
The safest baseline is:
deny all
allow specific flows that the business needs
Default allow is common in home networks and dangerous in flat corporate networks. Every unnecessary allowed path is an available one.
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