What is an ACL?

An Access Control List is an ordered set of permit/deny rules that filter traffic by criteria like IP address, port, or protocol.

ACL = Access Control List

Rules that filter network traffic based on criteria like IP addresses, ports, or protocols.

How it works:

IF source=192.168.1.0/24 AND destination_port=80 THEN PERMIT
IF source=ANY AND destination_port=23 THEN DENY

Where used:

• Router interfaces
• Firewalls
• Switch ports

Two types:

• Standard ACL: Filters by source IP only
• Extended ACL: Filters by source, destination, port, protocol

Memory tip: Read it literally — a list that controls access, evaluated top-down until a rule matches.

Go deeper:

• Access-control list (Wikipedia) — the "Networking ACLs" section: ordered permit/deny rules on routers/switches filtering by IP/port.