The Telecommunication Network Protocol (Telnet) enables a terminal to log in remotely to any device which is capable of operating as a telnet server, and provides an interactive operational interface via which the user can perform operations, in the same manner as is achieved locally via a console connection. Remote hosts need not be connected directly to a hardware terminal, allowing instead for users to take advantage of the ubiquitous capabilities of IP in order to remotely manage devices from almost any location in the world.
Telnet operates on a client/server model principle for which a telnet TCP connection is established between a user port and the server telnet port, which by default is assigned as port 23. The server listens on this well-known port for such connections. A TCP connection is full duplex and identified by the source and destination ports. The server can engage in many simultaneous connections involving its well-known port and user ports that are assigned from a non wellknown port range.
The telnet terminal drivers interpret the keystrokes of users and translates these to a universal character standard, based on a network virtual terminal (NVT) which operates as a form of virtual intermediary between systems, following which the transmission via the TCP/IP connection to the server is performed. The server decodes the NVT characters and passes the decoded characters to a pseudo terminal driver which exists to allow the operating system to receive the decoded characters.
Access to the telnet service commonly involves authentication of the user before access is granted. There are three main modes that are defined for telnet authentication.
Establishment to a device operating as a telnet server commonly uses a general password authentication scheme which is used for all users connecting to the user vty interface. Once IP connectivity is established through the implementation of a suitable addressing scheme, the authentication-mode password command set is implemented for the vty range, along with the password to be used
Following configuration of the remote device that is to operate as a telnet server, the client is able to establish a telnet connection through the telnet command, and receive the prompt for authentication. The authentication password should match the password implemented on the telnet server as part of the prior password authentication configuration. The user will be then able to establish a remote connection via telnet to the remote device operating as a telnet server and emulate the command interface on the local telnet client.
SUMMARY
If a user is unable to establish a telnet connection, the user should verify the device supporting the telnet service is reachable. If the device can be reached, the password should be verified. If the password is considered to be correct, the number of users currently accessing the device via telnet should be checked. If it is necessary to extend the number of users accessing the device through telnet, the user-interface maximum-vty <0-15> command should be used, where 0-15 denotes the number of supported users.