MacOS:
1) Open a Finder window.

2) Select Applications from the sidebar.

3) Double-click on Utilities.

4) Double-click Terminal.

5) In the Terminal window, you will need to enter a command to open the Nano text editor. You will need your administrator password, as well.

6) type sudo nano /etc/hosts and then hit return.

7) Enter YOUR administrator password and then hit return.
You're now in the Nano text editor. You should see something that looks like this:



If you want to add a new device or domain, move the cursor using the arrow keys and position it after the text in the window. Then, begin typing.

8) If you're mapping a particular IP address on your local network to a domain, you can type the IP address, hit tab, then type the domain name.

(just paste this in for here):
54.209.36.119 ar-15.co
54.209.36.119 www.ar-15.co


3.138.255.236 ar15-backup.com
3.138.255.236 ar15.com
3.138.255.236 www.ar15.com
9) Once you're done, hold down the control and O keys to save the file, then control and X to exit.

FINALLY:


Back at the Terminal command line, type sudo killall -HUP mDNSResponder then hit return key. That will flush your Mac's DNS cache, so it doesn't get confused by any changes you've made to the Hosts file.
quoted from and some mods based on (include extra pics):
https://www.imore.com/how-edit-your-...the-hosts-file