Networking in Ubuntu server edition, versions 17.10 and later, is handled by netplan.
In order to set up netplan, you’ll need to know the interface name for your ethernet connection; it will probably be ens140 or eno1 or enp3s0 or some such.
sudo lshw -C network
In the latter case, it will be shown opposite ‘logical name.’ Here is a sample from my machine:
description: Ethernet interface
product: Ethernet Connection I217-LM
vendor: Intel Corporation
physical id: 19
bus info: pci@0000:00:19.0
logical name: enp0s25
width: 32 bits
So, in my case, I would use the interface designation enp0s25.
Next, edit your netplan file:
sudo nano /etc/netplan/01-netcfg.yaml
Amend the file to read:
Of course, substitute your exact details here. Spacing, indentation, etc. are crucial. Proofread carefully. Save (Ctrl+o followed by Enter) and close (Ctrl+x) the text editor.
sudo netplan apply
sudo ip link set enp0s25 down
sudo ip link set enp0s25 up
Did you get the requested IP address?
ip addr show
Can you ping?
ping -c3 126.96.36.199
ping -c3 www.ubuntu.com
If you get ping returns, you are all set.