@farhan@mastodon.technology In Ubuntu 18.04 it's in Settings (e.g. Click Top right corner, Click on WiFi name opening triangle, click on Wifi settings) then click on the gear for your WiFi , go to the IPv4 tab, click on the radio button which says Manual and fill the details in. In XFCE (Xubuntu) DE something similar is under Applications > Settings > Advanced Network Settings.