When we want to configure some of the networking devices, we usually connect to the device with laptop, directly with console cable. This is the best way to set up the device for the first time, and it may be the only way. It’s because, first thing’s first, we need to setup the device to make it possible for us to communicate with other devices on the network. And then, when the router or the switch are ready to communicate, the best way to make all future changes is by doing it from a certain remote centralized location,by simply connecting to the devices using Telnet or SSH. This is because we don’t need to be literally in front of the device to configurate it; we can be on the other side of the network and remotely connect.
The TCP/IP Telnet app allows a terminal to remotely communicate with a device. Telnet with IP network sends and receives the data remotely, rather than a specialized cable and physical port on the device that is the case when we use directly connected computer with the device in first configuration setup for not connected device. The Telnet application protocols call the terminal emulator a Telnet client and the device that listens for commands and replies to them a Telnet server. Telnet is a TCP-based application layer protocol that uses port 23.
To be able of using Telnet, the user must install Telnet client software onto computer. The switch or router runs Telnet server software by default, but the switch or router does need to have an IP address configured so that it can send and receive IP packets. (You can see this configuration in our basic network device configuration section.) In addition, the network between the computer and router needs to be set up and working correctly so that the PC and switch can exchange IP packets and make connection.
Telnet sends all packets of data trough IP network as clear-text. That’s including the username and password to. This can be in most cases a potential security risk that is very easy to abuse. Many network technicians have the habit to use Telnet for monitoring switches or routers. They must have in mind the security risk behind Telnet connection to the device. On the other hand device administrator can be at his or her desk without having to go on the other part of the building or world and still get into the command line interface of that particular device.
Secure Shell (SSH)
Secure Shell (SSH) does the same basic things as Telnet, but in a more secure manner by using encryption of data packets that are crossing the network. Like the Telnet model, the SSH client software includes a terminal emulator and sends and receives the data using network. SSH uses port 22 instead of 23. As with Telnet, the SSH server, mostly on router or switch, receives the text from each SSH client, processes the text as a command, and sends messages back to the client. The difference between Telnet and SSH is simply in the fact that all the data packets are encrypted and in that case are private and less disposed to security risk.