Do you want your computer to treat a domain as localhost? You can map it as such in your /etc/hosts file. For example, if I have an web app that refers to itself with the dev.app.com domain, I can add the following line to my /etc/hosts file to make sure the domain resolves to localhost:

127.0.0.1 dev.app.com

Now, if I pop open my browser and visit dev.app.com:3000, I will see whatever is being served to localhost:3000.

h/t Chris Erin