Once a router is installed on a network, access to it is almost always through Telnet sessions, not through the console or AUX ports. Telnet is a technique to connect to a router as a virtual terminal. "Virtual" here means that a real terminal connection is not the device through a direct cable or a modem, as the console or AUX ports. Telnet connections are made instead of through the network. In most basic terms, a real terminal session consists of bits streaming one to one over a serial line. A virtual terminal session consists of IP packets routed over the network, pretending to be a continuous flow of bits over a serial line.

Telnet is a network application, it is not terminal emulator. It was buildup in the early days of UNIX OS as a way to connect to remote computers to manage them. Later, internet pioneers Telnet work included directly into the TCP / IP protocol networks as a way to get to the Internet and the device manager job. Telnet ships with every copy of Cisco IOS software and more computer OS.

When you use Telnet to access a router will perform so in a virtual line give by the Cisco IOS software. These are called VTYlines.Don't let the word "line" confuse you. It refers to a real communications circuit; it means a virtual terminal session in IOS Software. IOS supports up to five virtual terminal lines (numbered VTY 0-4, inclusive), which allows to have up to five virtual terminal sessions running on a router simultaneously. This is perhaps the design exaggeration, though. It is unusual to have more than one virtual terminal session running on a router simultaneously. A few routers have more than five VTY lines, and the Enterprise version of IOS, you can allocate even more than that.

Cisco IOS software used mostly character-based interface mode, which means that there is a point-and-click GUI environment, as we used to use our Microsoft Windows computers, Mac computers of Apple, or X-Windows UNIX workstations. If you connect to a router via the console port, AUX, or Telnet, will be delivered in the character-based IOS software interface. The following shows character-based IOS output:

The previous example is a record of the seven IOS lines—confor console, auxfor auxiliary, and vtyfor the virtual terminal. The seven lines are

* The console port, accessed through a local cable connection

* The AUX port, accessed through a modem connection

* Five VTY lines, accessed through TCP/IP network connections