ACL operation
ACLs are rules created by a network professional on the router or firewall appliance to filter traffic either entering or leaving the device. ACLs are a list of security rules, with each ACL containing either a permit or deny statement. Each statement within an ACL is referred to as an Access Control Entry (ACE). These ACEs are the real workers that allow and block packets between networks. When a router receives packets on an interface, the router checks each ACE, starting with the first entry at the top of the list and moving down until a match is found. Once a matching ACE is found, the router stops searching and executes the rule on the ACE, either permitting or denying the traffic. This process is known as packet filtering.
Important note
If no matches are found in the ACLs, the packet is discarded by the router. The last ACE within all ACLs is an implicit deny statement. An implicit deny statement simply states that if no matches are found in the previous...
