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Cisco ASA and PIX Firewall Handbook by Dave Hucaby

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6-3. Controlling Access with Access Lists

On a Cisco firewall, you can use access lists to filter traffic coming into or out of a firewall interface. Access lists that are applied to interfaces become an integral part of the traffic inspection mechanism.

Access lists can be defined using the familiar Cisco IOS software ACL format. However, one important difference exists between the firewall and IOS ACL formats: Firewalls use real subnet masks (a 1 bit matches, and a 0 bit ignores), whereas IOS platforms use a wildcard mask (a 0 bit matches, and a 1 bit ignores). Each line of an access list is an ACE.

Cisco firewalls also offer an ACL configuration feature not found in the Cisco IOS software. Access lists can be configured in a modular fashion, ...

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