Router Overview - Telnet
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Once a router is installed on a network, access to it is almost always made via Telnet sessions, not via the console or AUX ports. Telnet is a way to log into a router as a virtual terminal. “Virtual” here means that a real terminal connection is not made to the device via a direct cable or a modem, as with the console or AUX ports. Telnet connections are instead made through the network. In the most basic terms, a real terminal session is composed of bits streaming one by one over a serial line. A virtual terminal session is composed of IP packets being routed over a network, pretending to be bits streaming over a serial line.
Telnet is a network application, not a terminal emulator. It was developed in the early days of the UNIX operating system as a way to log into remote computers to manage them. Later, internetwork pioneers incorporated Telnet directly into the TCP/IP networking protocol as a way to get to and manage internetwork devices. Telnet ships with every copy of Cisco’s IOS software and most computer operating systems.
When using Telnet to access a router, you do so over a virtual line provided by the Cisco IOS software. These are called VTYlines.Don’t let the word “line” confuse you. It does not refer to an actual communications circuit; it means a virtual terminal session inside the IOS software. IOS supports up to five virtual terminal lines (numbered VTY 0–4, inclusive), making it possible to have up to five virtual terminal sessions running on a router at the same time. This is probably design overkill, however. It’s rare to have more than one virtual terminal session running on a router at the same time. Some routers have even more than five VTY lines, and with the Enterprise version of the IOS, you can even assign more than that.
Cisco’s IOS software is used mostly in character-based interface mode, which is to say that it’s not a point-and-click GUI environment such as we’ve grown accustomed to using on our Microsoft Windows PCs, Apple Macs, or X-Windows UNIX workstations. Whether logging into a router via the console port, AUX port, or Telnet, you are delivered to the character-based IOS software interface. The following shows character-based IOS output:
! line con 0
exec-timeout 0 0
line aux 0 transport input all
line vty 0 2
exec-timeout 0 0
password 7 1313041B
login
line vty 3
exec-timeout 5 0
password 7 1313041B
login
line vty 4
exec-timeout 0 0
password 7 1313041B
login
!
The preceding example is a listing 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
The HTTP Server User Interface A more recent router access method is HTTP Server. Don’t be misled by the name; no computer server is involved in using HTTP Server. The “server” in HTTP Server refers to a small software application running inside the Cisco IOS software. HTTP Server first became available with IOS Release 10.3. HTTP Server makes it possible to interact with the router through a Web browser. Figure 4-7 shows an HTTP Server screen.
Using HTTP to handle IOS command-line input and output isn’t particularly ergonomic. The majority of network administrators still prefer using the IOS software in character-based mode because it’s faster and more direct than pointing and clicking. This is not unlike those old hands who jump into Microsoft Windows’ MS-DOS prompt window to type in sys-tem-level commands, but Cisco may gradually move IOS toward a graphical user interface for a couple of reasons. The most obvious reason to at least offer a GUI-based alternative to working with IOS is that Cisco devices are increasingly being tended to by nonexperts. The other is that as complexity increases, the need for system visualization even inside a single router grows. Using visualization tools (to show load conditions, isolate errors, and so on) will, of course, require a browser instead of the old-fashioned “green screen” character-based command-line interface.
NOTE: To use the command-line interface, you must know what commands to type. You may want to use HTTP Server to get started and phase over to character-based mode as you become comfortable with the IOS command structure.
 This chapter is from Cisco: A Beginner's Guide, by Velte and Velte (McGraw-Hill/Osborne, 2004, ISBN: 0072256354). Check it out at your favorite bookstore today.
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