Full form: Access Control List
Also known as: Access Control List, access list, access-list
Quick Definition
An ordered list of permit/deny rules applied to a router interface to filter network traffic.
ACLs filter traffic based on source IP, destination IP, protocol, and port number. They are applied to a router interface in a direction: inbound (traffic entering the interface) or outbound (traffic leaving the interface). Standard ACLs (numbered 1–99) filter by source IP only and are applied close to the destination. Extended ACLs (numbered 100–199) filter by source, destination, protocol, and port, and are applied close to the source. ACLs process rules in order and stop at the first match; an implicit 'deny all' is always at the end.
access-list 10 permit 192.168.1.0 0.0.0.255 access-list 100 permit tcp 10.0.0.0 0.0.0.255 any eq 80 interface GigabitEthernet0/0 ip access-group 100 in
The implicit 'deny all' at the end of every ACL is invisible but always present. Forgetting it is a common mistake. Also, ACL rules are checked in sequence — more specific rules must come before general rules.
ACLs filter traffic based on source IP, destination IP, protocol, and port number. They are applied to a router interface in a direction: inbound (traffic entering the interface) or outbound (traffic leaving the interface). Standard ACLs (numbered 1–99) filter by source IP only and are applied close to the destination. Extended ACLs (numbered 100–199) filter by source, destination, protocol, and port, and are applied close to the source. ACLs process rules in order and stop at the first match; an implicit 'deny all' is always at the end.
The implicit 'deny all' at the end of every ACL is invisible but always present. Forgetting it is a common mistake. Also, ACL rules are checked in sequence — more specific rules must come before general rules.
ACL falls under the Security domain of the 200-301 exam. Understanding it in context with related terms like wildcard-mask and nat is essential for answering scenario-based questions correctly.