It is important to understand that Cisco devices, routers, and switches have several types of interfaces associated with them. You have worked with these interfaces in the labs. These interfaces, also commonly called ports, are where cables are connected to the device. See the figure for some example interfaces.
LAN Interfaces - Ethernet
The Ethernet interface is used for connecting cables that terminate with LAN devices such as computers and switches. This interface can also be used to connect routers to each other. This use will be covered in more detail in future courses.
Several conventions for naming Ethernet interfaces are popular, including AUI (older Cisco devices using a transceiver), Ethernet, FastEthernet and Fa 0/0. The name used depends on the type and model of the device.
WAN Interfaces - Serial
Serial WAN interfaces are used for connecting WAN devices to the CSU/DSU. A CSU/DSU is a device used to make the physical connection between data networks and WAN provider's circuits.
Serial interfaces between routers will also be used in our labs as part of various courses. For lab purposes, we will make a back-to-back connection between two routers using serial cables, and set a clock rate on one of the interfaces.
You may also need to configure other Data Link and Physical layer parameters on a router. To establish communication with a router via a console on a remote WAN, a WAN interface is assigned a Layer 3 address (IPv4 address).
The console interface is the primary interface for initial configuration of a Cisco router or switch. It is also an important means of troubleshooting. It is important to note that with physical access to the router's console interface, an unauthorized person can interrupt or compromise network traffic. Physical security of network devices is extremely important.
Auxiliary (AUX) Interface
This interface is used for remote management of the router. Typically, a modem is connected to the AUX interface for dial-in access. From a security standpoint, enabling the option to connect remotely to a network device carries with it the responsibility of maintaining vigilant device management.