What is an Access Control List (ACL)?
A commonly used tool at the Cisco command line is the access control list (ACL). At their simplest, access control lists are collections of IP addresses that are used by a router, switch, or a firewall to identify network traffic that must be handled in a special way. Cisco and other network vendors use ACLs for many different purposes. This article focuses on IOS access control lists as used on Cisco routers, although much of this discussion applies to Cisco switches as well.