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Security Fundamentals & Crypto · Foundational

Access Control List (ACL)

Ordered list of rules attached to an object or interface that permits or denies access to specific subjects or traffic.

Definition

Filesystem ACLs list users or groups and their permissions on an object. Network ACLs on routers and firewalls match packets by addresses, ports, and protocols, processed top-down with first match winning and an implicit deny at the end. Cloud platforms use network ACLs and security groups similarly.

Example

access-list 101 permit tcp any host 10.1.1.10 eq 443 permits HTTPS to one server on a Cisco router.

Exam tip

Rule order matters: put specific rules before general ones; anything unmatched hits the implicit deny.

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