CentOS 7虚拟机网卡启动不了解决办法
在VMMare12里安装了CentOS7虚拟机，但是开机发现连不了网，ifconfig一下，发现网卡没有启动。
eno16777736: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
ether 00:0c:29:2b:c0:1e txqueuelen 1000 (Ethernet)
RX packets 1 bytes 251 (251.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
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 0 (Local Loopback)
RX packets 386 bytes 33512 (32.7 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 386 bytes 33512 (32.7 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
习惯性的重启网卡服务“service network restart”
提示：
Restarting network (via systemctl): [ OK ]
重启OK，，，，
再查看网卡"ifconfig"
还是老样子
于是"ifup eno16777736"手动重启网卡
提示：
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/0)
启动成功
再查看网卡状态"ifconfig"
eno16777736: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.41.128 netmask 255.255.255.0 broadcast 192.168.41.255
inet6 fe80::20c:29ff:fe2b:c01e prefixlen 64 scopeid 0x20<link>
ether 00:0c:29:2b:c0:1e txqueuelen 1000 (Ethernet)
RX packets 74 bytes 9637 (9.4 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 61 bytes 7083 (6.9 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 0 (Local Loopback)
RX packets 650 bytes 56612 (55.2 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 650 bytes 56612 (55.2 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
OK，，得到IP问题解决。
后续：
本来这个问题不会出现的，主要是我的网络环境是有DHCP服务器，IP地址可以自动获得。如果是手动输入IP就不会出现这个问题了，因为在网卡配置文件里有一项"ONBOOT=no"表示引导时不激活网卡，由于对centos7不熟悉，所以未发现这个问题。
上面的解决方法是不彻底的，因为每重启一次服务器就需要输入一次"ifup eno16777736",很是麻烦，我们可以修改网卡的配置文件"/etc/sysconfig/network-scripts/ifcfg-eno16777736"里面的"ONBOOT=no"修改成"ONBOOT=yes",至此问题彻底解决！