Telnet 是一种网络协议，用于在本地计算机和远程计算机之间建立一个虚拟终端会话。它允许用户在本地计算机上运行远程计算机的命令行界面

1.安装telnet

yum install -y telnet
apt-get  install telnet

2.探测端口联通性

2.1端口被封，有墙堵住
[root@DZSWJ_FZJH1 ~]# telnet 86.103.72.41 80
Trying 86.103.72.41...

2.2端口开放，服务未启动
[root@WWMYSQL ~]# telnet 10.104.0.10 10051
Trying 10.104.0.10...
telnet: connect to address 10.104.0.10: Connection refused

2.3端口开放，服务也开启
[root@WWMYSQL ~]# telnet 10.104.0.12 22
Trying 10.104.0.12...
Connected to 10.104.0.12.
Escape character is '^'.
SSH-2.0-OpenSSH_7.3