ogin to apply an authentication method list. Here, I'm applying the GAD-Login method list cre- ated earlier to VTY lines 0-4. This will have the effect of challenging Telnet sessions to the router with whatever authentication methods exist in the GAD-Login method list: line vty o 4 login authentication GAD-Login To apply a PPP authentication method list to an interface, you must configure the interface with PPP encapsulation. The ppp authentication command is used to enable authentication. The authentication protocol must be specified along with the method list. Here, I have specified CHAP along with the method list GAD-Login: Chapter 26: Authentication in Cisco Devices interface Serial0/0/0:0 no ip address encapsulation ppp ppp authentication chap GAD-Login If you have not created a PPP method list, you will get an error, though the command will still be accepted: R1 (config-if)#ppp - authentication pap GAD-Login AAA: Warning, authentication list "GAD-Login" is not defined for PPP. Using AAA with PAP or CHAP is much more scalable than using locally configured usernames and passwords. AAA Authentication CHAPTER 27 Basic Firewall Theory A firewall is the wall in a car that protects you from harm when the engine catches fire. At least, that's the definition that confused my mother when I told her I was writing this chapter. In networking, a firewall is a device that prevents certain types of traffic from entering or leaving your network. Usually, the danger comes from attackers at- tempting to gain access to your network from the Internet, but not always. Firewalls are often deployed when networks are connected to other entities that are not trusted, such as partner companies. A firewall can be a standalone appliance, software running on a server or router, or a module integrated into a larger device, like a Cisco 6500 switch. These days, a firewall's functionality is often included in other devices, such as the ubiquitous cable-modem/ router/firewall/wireless access point devices in many homes. Modern firewalls ca