You'll do this lab on your Ubuntu virtual machine.

1. Shut down your Ubuntu virtual machine, and create a snapshot. 

2. Look at your iptables rules, or lack thereof, with:

        sudo iptables -L

3. Create the rules that you need for a basic firewall, allowing for Secure Shell access but denying everything else:

        sudo iptables -A INPUT -m conntrack 
                                      --ctstate ESTABLISHED,RELATED
                      -j ACCEPT
        sudo iptables -A INPUT -p tcp --dport ssh -j ACCEPT
        sudo iptables -A INPUT -j DROP

4. View the results with:

        sudo iptables -L

5. Oops, it looks like you forgot about that loopback interface. Add a rule for it at the top of the list:

        sudo iptables -I INPUT 1 -i lo -j ACCEPT

6. View the results with these two commands. Note the difference between the output of each:

        sudo iptables -L
        sudo iptables -L -v

7. Install the iptables-persistent package, and choose to save the IPv4 and IPv6 rules when prompted:

         sudo apt install iptables-persistent

8. Reboot the virtual machine and verify that your rules are still active.
9. End of lab.

Now, I know you're thinking, "Wow, that's a lot of hoops to jump through just to set up a basic firewall." And yeah, you're right. So, give me a moment to get rid of what I just did with iptables, and I'll show you what the Ubuntu folk came up with to make things simpler.