[root@linux-node2~]# reboot                           #必须重启系统生效
[root@linux-node2~]# yum install net-tools                   #默认centos7不支持ifconfig 需要看装net-tools包
[root@linux-node2~]# ifconfig eth0                 #在次查看网卡信息
eth0:flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500inet 192.168.56.12 netmask 255.255.255.0 broadcast 192.168.56.255inet6 fe80::20c:29ff:fe5c:7bb1 prefixlen 64 scopeid 0x20<link>ether 00:0c:29:5c:7b:b1 txqueuelen 1000 (Ethernet)RX packets 152 bytes 14503 (14.1 KiB)RX errors 0 dropped 0 overruns 0 frame 0TX packets 98 bytes 14402 (14.0 KiB)TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 

[root@localhost network-scripts]# service NetworkManager stop
Starting network (via systemctl):  Job for network.service failed because the control process exited with error code. See "systemctl status network.service" and "journalctl -xe" for details.[root@localhost network-scripts]# systemctl status network.service
?.network.service - LSB: Bring up/down networkingLoaded: loaded (/etc/rc.d/init.d/network)Active: failed (Result: exit-code) since Mon 2017-03-20 18:41:03 CST; 3min 49s agoDocs: man:systemd-sysv-generator(8)Process: 18880 ExecStart=/etc/rc.d/init.d/network start (code=exited, status=1/FAILURE)Mar 20 18:41:03 localhost.localdomain network[18880]: RTNETLINK answers: File exists
Mar 20 18:41:03 localhost.localdomain network[18880]: RTNETLINK answers: File exists
Mar 20 18:41:03 localhost.localdomain network[18880]: RTNETLINK answers: File exists
Mar 20 18:41:03 localhost.localdomain network[18880]: RTNETLINK answers: File exists
Mar 20 18:41:03 localhost.localdomain network[18880]: RTNETLINK answers: File exists
Mar 20 18:41:03 localhost.localdomain network[18880]: RTNETLINK answers: File exists
Mar 20 18:41:03 localhost.localdomain systemd[1]: network.service: control process exited, code=exited status=1
Mar 20 18:41:03 localhost.localdomain systemd[1]: Failed to start LSB: Bring up/down networking.
Mar 20 18:41:03 localhost.localdomain systemd[1]: Unit network.service entered failed state.
Mar 20 18:41:03 localhost.localdomain systemd[1]: network.service failed.原因分析和解决：
在centos下出现该故障的原因是启动网络的两个服务/etc/init.d/network 和 /etc/init.d/NetworkManager有冲突。
从根本上说是NetworkMaganager（NM）的带来的冲突，停用NetworkManager即可解决。重启即可。
[root@localhost network-scripts]# service NetworkManager stop
Redirecting to /bin/systemctl stop  NetworkManager.service
[root@localhost network-scripts]# systemctl start network.service
[root@localhost network-scripts]#