The hosts file is an OS based plain text file that maps hostnames to IP addresses.It contains lines of IP address in the first field and host names in the second field. Each field is separated by tab space. you can also add a comment line by adding a hash (#) in the front.
Edit the hosts file on Ubuntu:
Open your Terminal app searching “terminal” in the unity dash or by press Ctrl+Alt+T key combination. If you want to take a backup of your hosts file , typing the following command in your terminal app.
sudo cp /etc/hosts /etc/hosts.old
Type your ubuntu password and press enter. Then run the below command in terminal.
sudo gedit /etc/hosts
sudo -E gedit /etc/hosts
It will open the hosts file in the gedit text editor.
It you you want to block www.google.com, then add below line in your hosts file.
Save the file and close it. You can add as many sites as you want.
If the blocking is not working correctly in Google chrome browser, then just disable the “Use a web service to help resolve navigation errors” option in the chrome settings.