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You didn't say which distro you are using. 
I use Debian based Distros and configure a file named /etc/network/interfaces. 
Here is an example of one of my network interfaces files.  
                 ---------------------------------------------- 
# This file describes the network interfaces available on your system 
# and how to activate them. For more information, see interfaces(5). 
auto lo  
# The loopback network interface 
iface lo inet loopback 
 
auto eth0 
# The primary network interface 
iface eth0 inet dhcp 
 
            ---------------------------------------------------- 
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