Historically, Telnet provided access to a command-line interface on a remote host. However, because of serious security concerns when using Telnet over an open network such as the Internet, its use for this purpose has waned significantly in favor of SSH.
Telnet is basically used for accessing the network device remotely for management purposes. because it’s not possible for us to stay in front of the device to take access to the device via console cable .
VTY stand for virtual Tele Type . Telnet uses port 23 .
Telnet is a client-server protocol, based on a reliable connection-oriented transport. Typically, this protocol is used to establish a connection to Transmission Control Protocol (TCP) port number 23, where a Telnet server application (telnetd) is listening.
The VTY lines are the Virtual Terminal lines of the router, used solely to control inbound Telnet connections. They are virtual, in the sense that they are a function of software – there is no hardware associated with them.
Telnet, by default, does not encrypt any data sent over the connection (including passwords), and so it is often feasible to eavesdrop on the communications and use the password later for malicious purposes; anybody who has access to a router, switch, hub or gateway located on the network between the two hosts where Telnet is being used can intercept the packets passing by and obtain login, password and whatever else is typed with a packet analyzer.
Switch(config)# line vty 0 4 Switch(config-line)# password cisco Switch(config-line)# login OR Switch(config)# username admin privilege 15 password cisco Switch(config-line)# login local