when you type
on ubuntu server you will find this line
auto eth0 iface eth0 inet dhcp.
It means that the network interface has been set up to automatically reseove to DHCP(The Dynamic Host Configuration Protoco). If you are setting up a Domain name server -BIND.
if you see somthing like this which you are likely to see in accounts hosted on cpanels or on digitalocean
iface eth0 inet static address 192.168.1.5 netmask 255.255.255.0 gateway 192.168.1.254
It means the physical interfaces which is the eth0 has been defined and is static.