Difference between Telnet & SSH

Secure Shell ( SSH), and Telnet are two network protocols that have been used to communicate with remote routers , servers and switches.  The primary difference is that all data  sent to the remote host , including password  is transmitted in clear text , and that's a big security issue.
SSH offers security mechanisms that protect the users against anyone with malicious intent while Telnet has no security measures whatsoever and can cause all kind of trouble in our network.

You can see the options when you connect your computer to a switch or router:

Telnet

Telnet was designed to work within a private network and not across a public network where threats can appear. Because of this, all the data is transmitted in plain text, including passwords. This is a major security issue and the developers of SSH used encryption to make it harder for other people to sniff the password and other relevant information.
Telnet also omits another safety measure called authentication. This ensures that the source of the data is still the same device and not another computer. Without authentication, another person can intercept the communication and do what he wishes. This is also addressed in SSH as it uses a public key to authenticate the source of the data.

SSH (Secure Shell)

Secure Shell frequently known as SSH is a network protocol which is used to form a reliable and stable connection among two remote computers within the network or on the internet. It uses a  an encrypted form to transmit data between computers. This obscure language or code allows confidentiality of the information that has been swapped.

A lot of people  use of SSH for the purpose of login to remote servers and performing remote commands and instructions because it contains high security. With the use of this system, any user can transfer his private details like user id, passwords, and other personal information in a protective way. As these data uses hidden format, and the hackers are not able to read them. It can also be used in public networks. It gives a kind of secure transmission over insecure paths.

Due to the security measures that were necessary for SSH to be used in public networks, each packet contains less data to make room for the data of the security mechanisms. In order to transmit the same amount of data , you would need to take-up a lot more bandwidth. This is called overhead and was such a major issue back when internet speeds were very low because it translates to a performance hit.

The security issues of Telnet forced a lot of people to use SSH in order to protect themselves. It didn’t take a long time before SSH replaced Telnet in a great majority of its uses. Telnet did not fade away though as it is still used in some areas, mostly in testing and debugging. Telnet extensions were developed to provide security but they are not used in most Telnet implementations.

Summary:

1. SSH and Telnet commonly serves the same purpose
2. SSH encrypts the data while Telnet sends data in plain text
3. SSH uses a public key for authentication while Telnet does not use any authentication
4. SSH adds a bit more overhead to the bandwidth compared to Telnet
5. SSH is more secure compared to Telnet
6. Telnet has been all but replaced by SSH in almost all uses