Answer :

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The obvious answer is Layer 3, the network layer. but it's not always the case.

Point in case. Since firewalls essentially filter data, the answer depends on what kind of filtering you do.

If you filter based on IP address (for example), you can say that your firewall is filtering at layer 3. If you filter specific ports, you can say you're filtering at layer 4. If your firewall inspects specific protocol states or data, you can say it operates at layer 7.

The truth is that most firewalls do all these things in combination. So practically speaking there really is no useful answer to your question.

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