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> What Are ACLs?
Cisco routers can be used as part of a good overall security strategy. One of the most important tools in Cisco IOS software used as part of that strategy are access control lists (ACLs). ACLs define rules that can be used to prevent some packets from flowing through the network. The most typical use of ACLs on the CCNA exams is to filter packets.
Cisco IOS has supported IP ACLs almost since the original commercial Cisco routers were introduced in the late 1980s. IOS identified these ACLs with a number. Years later, as part of the introduction of IOS 11.2, Cisco added the ability to create named ACLs. These named ACLs provide some other minor benefits as compared to numbered ACLs, but both could be used to filter the exact same packets with the exact same rules. Finally, with the introduction of IOS 12.3, Cisco improved ACL support again, particularly with how IOS allows engineers to edit existing ACLs. This latest major step in the progression of ACLs over the years makes numbered and named ACLs support the exact same features, other than the one obvious difference of using either a number or a name to identify the ACL.
Odom, Wendell (2008) CCNA ICND2 Official Exam Certification Guide, Second Edition. Indianapolis: Cisco Press.