Some Network Protocols

Telnet

Telnet is an application protocol that allows a user to communicate with a remote device. A user on a client machine can use a software (known as a Telnet client) to access a command-line interface of another, remote machine that is running a Telnet server program.

Telnet is often used by network administrators to access and manage  devices. A network administrator can access the device by telnetting to the IP address or hostname of a remote device. The network administrator will then be presented with a virtual terminal that can interact with the remote host.

To use telnet, you must have a software (Telnet client) installed. On a remote device, a Telnet server must be installed and running.

Telnet uses a well-known TCP port 23 for its communication.

Although Telnet is simple and easy to use, it is not widely used anymore, especially in production environments. This is because Telnet sends all data in clear-text, including usernames and passwords!SSH is commonly used today instead of Telnet. Telnet is only used if SSH is not available on the device, for example on some older Cisco equipment.

Secure Shell (SSH) protocol

Just like Telnet, the Secure Shell (SSH) protocol enables a user to access a remote device and manage it. The key difference between Telnet and SSH is that SSH uses encryption. All data transmitted over a network (including usernames and passwords) is encrypted and secure from eavesdropping. SSH employs public key cryptography for the encryption.

A user on a client machine can use a software (an SSH client) to establish a connection to an SSH server running on a remote machine. Once the connection has been established, the user can execute commands on the remote device.


SSH uses a well-known TCP port 22.

File Transfer Protocol (FTP)

File Transfer Protocol (FTP) is a network protocol used to transfer files between machines on a TCP/IP network. FTP employs a client-server architecture; the client has an FTP client installed and establishes a connection to an FTP server running on a remote machine. After the connection has been established and the user is successfully authenticated, the data transfer phase can begin. Note that, although FTP does support user authentication, all data is sent in clear text, including usernames and passwords.

FTP uses two TCP ports: port 20 is used for sending data and port 21 for sending control commands.

Trivial File Transfer Protocol (TFTP)

Trivial File Transfer Protocol (TFTP) is a network protocol used to transfer files between hosts in a TCP/IP network. It is a simpler version of FTP and it doesn't have all of its functions; for example, you can not list, delete, or rename files or directories on a remote server. In fact, TFTP can only be used to send and receive files between the two computers. TFTP doesn't support user authentication and all data is sent in clear text.

The only real advantage that TFTP has over FTP is that it uses less resources.

TFTP uses a well-known UDP port 69.