When you insert a new network interface to a already setup server, it will not recognized your new added network interface correctly. However, we can edit it manually.
Step 1 :
sudo nano /etc/udev/rules.d/70-persistent-net.rules
Edit the MAC address and/or name of the interface when necessary.
Step 2 :
sudo nano /etc/network/interfaces
Add a new entry for the new interface accordingly.
auto eth2
iface eth2 inet dhcp
*dhcp if IP is assigned automatically by DHCP
Step 3 :
sudo ifconfig ethX up
sudo /etc/init.d/networking restart
*where ethX will be eth0, eth1, eth2 ….
