In section 3 (https://ubuntu.com/tutorials/how-to-install-ubuntu-on-your-raspberry-pi#3-wifi-or-ethernet):
If you want to use your local lan’s DHCP server to get an IP address for your pi then there are two ways to get your Pi connected to the internet:
The first is to connect your Pi to your router with an ethernet cable. In this case, you can skip this step and go to the next one in the tutorial.
The second requires a local Wi-Fi network that both your computer and Pi can be connected to. We are going to edit files you just downloaded on your SD card to ensure your Pi can connect to the Wi-Fi network at boot.
If you want to have the pi boot the pi to known IP address then see the section “Set a Static IP” below Wifi where you will edit the same file that the Wifi section tells you to modify
Set A Static IP
To set a static IP you need to replace the
line in the network-config file with lines that specify the intended IP address of the pi as well as its default gateway and DNS server. You can do this for either the eth0 or wlan0 interface (or both). It is important that you get the indenting right for this work correctly
For example if you were planning to give the pi the address 192.168.1.23 in the 192.168.1.0/24 subnet with a default gateway of 192.168.1.1 and a DNS server of 192.168.1.53 then the following text would work.