ACL
Access Control List
An ordered set of rules that permits or denies traffic or access to a resource.
An ACL is an ordered set of permit and deny entries evaluated top to bottom, where the first match wins and an implicit deny typically drops anything not explicitly allowed. Router and firewall ACLs filter by source, destination, port, and protocol, while filesystem ACLs grant rights to specific users or groups on objects. A frequent pitfall is rule ordering: a broad permit placed above a narrower deny can unintentionally expose traffic, so placement matters as much as the rules themselves.
Looking ACL up is step one. Getting tested on it is step two.
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