当使用mysql -uroot -p -h272.16.1.51 登录数据库的时候，用的是TCP协议，用status命令查看：

mysql> status;
--------------
mysql  Ver 14.14 Distrib 5.6.38, for Linux (x86_64) using  EditLine wrapper

Connection id:          10
Current database:
Current user:           root@mysql03
SSL:                          Not in use
Current pager:          stdout
Using outfile:            ''
Using delimiter:         ;
Server version:            5.6.38-log Source distribution
Protocol version:        10
Connection:                172.16.1.53 via TCP/IP
Server characterset:    utf8
Db     characterset:      utf8
Client characterset:     utf8
Conn.  characterset:    utf8
TCP port:               3306
Uptime:                 18 min 40 sec

而不加 h 的时候，是这样的：

mysql> status;
--------------
mysql  Ver 14.14 Distrib 5.6.38, for Linux (x86_64) using  EditLine wrapper

Connection id:          11
Current database:
Current user:           root@localhost
SSL:                         Not in use
Current pager:           stdout
Using outfile:           ''
Using delimiter:         ;
Server version:          5.6.38-log Source distribution
Protocol version:       10
Connection:                Localhost via UNIX socket
Server characterset:    utf8
Db     characterset:     utf8
Client characterset:     utf8
Conn.  characterset:    utf8
UNIX socket:              /application/mysql-5.6.38/tmp/mysql.sock
Uptime:                      21 min 36 sec

是使用的UNIX socket，即UNIX套接字。