Telnet 是一种网络协议，允许用户使用基于文本的界面通过网络与远程设备通信。它在早期的网络应用中被广泛用于远程管理和故障诊断，使用户能够连接到远程机器和服务，通常是通过 TCP/IP 网络。

Telnet is a network protocol that allows a user to communicate with a remote device over a network using a text-based interface. It was widely used in the early days of networking for remote management and troubleshooting, and it enables a user to connect to remote machines and services, typically over TCP/IP networks.

Protocol

* Telnet 协议是 TCP/IP 协议套件的一部分，默认情况下通过 TCP 端口 23 运行，但如果指定，也可以使用其他端口。

* 它主要用于建立远程连接，并使用文本命令与另一台机器通信。

* Telnet Protocol is part of the TCP/IP suite and operates over TCP port 23 by default, though it can use other ports if specified. 

* It is primarily used to establish a remote connection and communicate with another machine using text commands.

Functionality

* 命令行界面（CLI）： Telnet 会打开一个命令行会话，用户可在其中远程运行命令。

* 无加密： Telnet 的一个主要缺点是它以纯文本传输数据，包括用户名和密码，这使得它在现代网络中的大多数用途都不安全。

* Command-line Interface (CLI): Telnet opens a command-line session where a user can run commands remotely.

* No Encryption: One of the major downsides of Telnet is that it transmits data, including usernames and passwords, in plain text, making it insecure for most uses in modern networking.

Usage

Telnet 传统上用于

* 远程登录： 您可以登录另一台计算机，远程执行命令。

* 端口测试： 您可以使用 Telnet 检查远程服务器上的特定端口是否正在运行服务。例如，连接到 80 端口（HTTP）的服务器，就能知道网络服务是否在运行。

* 排除网络服务故障： 管理员使用 Telnet 手动连接和测试网络服务器、邮件服务器（SMTP、IMAP）等服务，从而排除网络服务的连接问题。

Telnet was traditionally used for:

* Remote Login: You can log into another computer to execute commands remotely.

* Port Testing: You can use Telnet to check if a service is running on a particular port on a remote server. For instance, connecting to a server on port 80 (HTTP) will let you know if the web service is active.

* Troubleshooting Network Services: Administrators use Telnet to troubleshoot connectivity issues with network services by manually connecting to and testing services like web servers, mail servers (SMTP, IMAP), etc.

How it Works

Telnet 在两台机器之间创建一个双向、面向文本的交互式通信会话。启动连接后，用户可以向远程机器发送命令，远程机器也会回发响应。

Telnet creates a bi-directional, interactive text-oriented communication session between two machines. After initiating the connection, the user can send commands to the remote machine, and the remote machine sends responses back.

Security Concerns

由于 Telnet 缺乏加密功能，在远程管理任务中，Telnet 基本上已被 SSH（安全外壳）所取代。SSH 对整个通信进行加密，使敏感数据的传输更加安全。

Due to its lack of encryption, Telnet has largely been replaced by SSH (Secure Shell) for remote management tasks. SSH encrypts the entire communication, making it much safer for transmitting sensitive data.

Telnet Client

如何使用： 要使用 Telnet，一般需要打开终端或命令提示符，并使用以下语法：

How to use: To use Telnet, you generally open a terminal or command prompt and use the following syntax:

telnet <hostname or IP address> <port>

For example:

telnet example.com 80

这将通过 80 端口（HTTP）连接到远程服务器 example.com。

This connects to the remote server example.com on port 80 (HTTP).

Telnet vs SSH

Telnet	SSH
Plaintext transmission (insecure)	Encrypted communication (secure)
Default port 23	Default port 22
Lacks strong authentication	Strong authentication and encryption
Largely deprecated	Widely used for secure remote access

Common Uses Today

尽管 Telnet 被认为对大多数任务不安全，但偶尔仍会用于以下情况：

* 内部隔离网络，安全问题较少。

* 测试网络服务，查看某项服务是否可连接并在某个端口上运行。

* 教育目的，了解网络基础知识。

Although Telnet is considered insecure for most tasks, it’s still occasionally used for:

* Internal, isolated networks where security is less of a concern.

* Testing network services to see if a service is reachable and running on a certain port.

* Educational purposes to understand networking basics.

How Do I Enable Telnet on Windows

Open "Control Panel" -> Uninstall a program -> Trun Windows features on or off ->Telnet Client

或直接Win+Q搜索“Windows features”。

然后在命令行中就可以输入telnet命令了。

Summary

Telnet 是访问远程设备的基本工具，但对于安全的远程管理任务来说已基本过时。它对于简单的测试和诊断仍然有用。

Telnet is a basic tool for accessing remote devices but is mostly obsolete for secure remote administration tasks. It remains useful for simple testing and diagnostics.

参考：

What Is Telnet? How Do I Run It? (intermedia.com)