An ACL is a list of access control entries. Each entry in the list will be a pair of a ‘who’ - something like a user or an address-range - who the entry applies to, followed by rules concerning things that are allowed or forbidden, or else levels of access permitted or some such. An ACL will apply to some object or other. In a file system, an ACL for a file might specify who is allowed to do what to that particular file. In a firewall, ACLs might specify the rules to be applied when certain types of packets are seen heading in one direction or another, with match conditions concerning source or destination addresses, ports and protocol types, and the ACL conditions might be checked at a particular interface.