Access List Tutorial

An access control list (ACL) is the primary tool a router uses to filter traffic. The idea is similar to a guard checking tickets at the entrance to an event: the ACL inspects each packet arriving at or leaving an interface and decides whether to let it through or block it, based on criteria the administrator defines. In other words, an ACL controls whether packets are forwarded or dropped at a router’s interface, according to rules written into the list.

Before an ACL can do anything, it has to be created and then applied to a specific interface and direction. There are three main types of IP access list: Standard, Extended, and Named.