默认安装centos7的时候，选择最小安装，这时候默认没有启用网络的，系统开机后无法使用网络
1.查看物理网卡的地址：
ifconfig 或者 ip addr都可以的
[root@localhost ~]# ifconfig
ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.80.136 netmask 255.255.255.0 broadcast 192.168.80.255
inet6 fe80::c89b:1871:b2c6:3a60 prefixlen 64 scopeid 0x20<link>
ether 00:0c:29:0e:a8:cb txqueuelen 1000 (Ethernet)
RX packets 1778 bytes 116068 (113.3 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 175 bytes 23426 (22.8 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1 (Local Loopback)
RX packets 268 bytes 23300 (22.7 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 268 bytes 23300 (22.7 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
2.可以看到ens33为物理网卡，这时候，可以用以下方式去开启网络连接
[root@localhost ~]# cd /etc/sysconfig/network-scripts/
[root@localhost network-scripts]# ll
总用量 232
-rw-r--r--. 1 root root 310 4月 11 2019 ifcfg-ens33
[root@localhost ~]# vi /etc/sysconfig/network-scripts/ifcfg-ens33
改成：onboot=yes
接ESC 退出 :wq 保存
3.重启网络服务:
[root@localhost network-scripts]# systemctl restart network.service
4.重启后：ping www.baidu.com
解析正常了，网络启用正常