After start up Arch Linux host, plugin the ethernet cable, the lights of network interface card is off. Now run command
ip link set enp0s25 up
the lights will turned on. Then run
dhcpcd enp0s25
Now the host will get a IP. If you can ping 8.8.8.8 (which is google's dns server), but not www.baidu.com, you have to assign a valid DNS provider. Using OpenDNS is a safe solution. So add the following lines into /etc/resolv.conf:
nameserver 208.67.222.222
nameserver 208.67.220.220