Cisco routers are used widely both on the Internet and in corporate intranets. At the same time, the Cisco Internet Operating System (IOS) has grown to be very large and complex, and Cisco documentation fills several volumes. focuses on a critical aspect of the Cisco IOS--access lists. Access lists are central to the task of securing routers and networks, and administrators cannot implement access control policies or traffic routing policies without them. Access lists are used to specify both the targets of network policies and the policies themselves. They specify packet filtering for firewalls all over the Internet. covers three critical areas:Intranets.Firewalls.The Internet. differs from other Cisco router titles in that it focuses on practical instructions for setting router access policies. The details of interfaces and routing protocol settings are not discussed.