Instructor Note   One purpose of telnet is described. Another purpose, common as a second semester troubleshooting tool, is to telnet into various routers. As a simple activity, you may want to have the students telnet into another computer via your LAN. This target indicator describes the details of the Telnet process. Emphasis is on the client-server model and the 7 OSI layers. This TI relates to CCNA Certification Exam Objectives #1.

Terminal emulation (Telnet) software provides the ability to remotely access another computer. It allows you to log in to an Internet host and execute commands. A Telnet client is referred to as a local host, and a Telnet server, which uses special software called a daemon, is referred to as a remote host.

To make a connection from a Telnet client, you must select a connection option. A dialog box prompts you for a "Host Name" and "Terminal Type". The host name is the IP address (DNS) of the remote computer to which you connect. The terminal type describes the type of terminal emulation that you want the computer to perform. The Telnet operation uses none of the transmitting computer’s processing power. Instead, it transmits the keystrokes to the remote host and sends the resulting screen output back to the local monitor. All processing and storage take place on the remote computer.  

Telnet begins with the e-mail process. When you enter a DNS name for a Telnet location, the name must be translated into its associated IP address before a connection can be established. The Telnet application works mainly at the top three layers of the OSI model - the application layer (commands), the presentation layer (formats, usually ASCII), and the session layer (transmits). The data then passes to the transport layer where it is segmented, and the port address and error checking are added. The data then passes to the network layer where the IP header (containing the source and destination IP addresses) is added. Next, the packet travels to the data link layer, which encapsulates the packet in a data frame, adds the source and destination MAC address, and a frame trailer. If the source computer doesn’t have the MAC address of the destination computer, it performs an ARP request. When the MAC address has been determined, the frame travels across the physical medium (in binary form) to the next device.

When the data reaches the remote host computer, the data link, network, and transport layers, reassemble the original data commands. The remote host computer executes the commands and transmits the results back to the local client computer by using the same process of encapsulation that delivered the original commands. This whole process repeats itself, sending commands and receiving results, until the local client has completed the work that needs to be done. When the work is done, the client terminates the session.

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	What is Telnet?