You can connect to a Cisco router to configure it, verify its configuration, and check statistics. There are different ways to do this, but most often, the first place you would connect to is the console port. The console port is usually an RJ-45 (8-pin modular) connection located at the back of the router--by default, there’s no password set.

You can also connect to a Cisco router through an auxiliary port--which is really the same thing as a console port, so it follows that you can use it as one. But this auxiliary port also allows you to configure modem commands so that a modem can be connected to the router. This is a cool feature—it lets you dial up a remote router and attach to the auxiliary port if the router is down and you need to configure it “out-of-band” (meaning “out-of-the-network”). “In-band” means the opposite—configuring the router through the network.
The third way to connect to a Cisco router is in-band, through the program Telnet. Telnet is a terminal emulation program that acts as though it’s a dumb terminal. You can use Telnet to connect to any active interface on a router, such as an Ethernet or serial port.

Figure as below shows an illustration of a 2501 Cisco router. Pay close attention to all the different kinds of interfaces and connections.