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.