|  |  | There are a few key points to keep in mind when creating the rules that will govern your packet filtering router. Place the most specific rules at the top of the ACL. Cisco routers operate on a first match basis. Cisco has an implicit deny command at the end of all rules; once you apply a rule, all else is implicitly denied. When using the command line interface, all new rules are applied at the end. It is proper to apply new rules at the end of existing rules; otherwise, new rules may replace the old.