The Mac OS X hosts file

Introduction

The hosts file...

doesn't have an extension, its just called hosts
is a text file that maps hostnames to IP addresses
is checked before the active network connection's DNS servers
is located at /etc/hosts, which is a symbolic link to /private/etc/hosts

Editing the hosts file

Editing the hosts file in Mac OS X is a pretty easy task, especially if you are familiar with the terminal.

1 Open the Terminal.app

Press ⌘-space and start typing terminal, or navigate to: Applications ➤ Utilities ➤ Terminal

2 Open the hosts file

Enter the following command in Terminal to open hosts in a text-editor:
sudo nano /etc/hosts
Enter your (admin) user password when prompted.

3 Edit the hosts file

The hosts file contains some comments (lines starting with the # symbol), as well as some default hostname mappings (one per line), entered as IP address followed by one or more hostnames (e.g. 127.0.0.1 localhost).
Append your new mappings underneath the existing ones. Navigate using the arrow keys, leave space between fields, and make sure there is an empty blank line after the last entry. Here we'll add ucm0.pbxbook.com:

We can also use the hosts file to block access to websites! Make an entry like: 127.0.0.1 facebook.com to locally block facebook (for example), or enter: 157.166.226.25 facebook.com to direct facebook requests to cnn.com.

4 Save the hosts file

When done editing the hosts file, press Ctrl-o to save the file.
Press enter on the filename prompt to leave it unchanged, and Ctrl-x to exit the editor.

5 Flush the DNS cache

Enter the following command (Big Sur) to flush the DNS cache and activate the changes:

sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

For older OS X versions, refer to: https://support.apple.com/en-ca/HT202516
You can now test your new mapping with ping mypbx.com, or by entering the URL in a browser window!
Use dig, or nslookup with a URL to see if it's resolved.

The standard hosts file:

If you need restore the hosts file, here's the original:

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting. Do not change this entry.
##
127.0.0.1       localhost
255.255.255.255 broadcasthost
::1 localhost
fe80::1%lo0     localhost

To restore, the above, open Terminal, and type: sudo pbpaste > /etc/hosts

How to make the internet not suck (as much):

You can use a custom hosts file as a simple ad-blocker while surfing.

http://somebodywhocares.org offers a custom hosts file as "an easy and effective way to protect you from many types of spyware, reduce bandwidth, prevent pop-up traps, prevent user tracking, and block most advertising you would otherwise be subjected to on the Internet."

Download their custom hosts file, rename and copy it to /etc/hosts.love
Install by swapping the active and inactive files (remember, the active file is always hosts):

sudo cp /etc/hosts /etc/hosts.original
sudo cp /etc/hosts.love /etc/hosts

Or, write the hosts file directly with curl, for example:

sudo curl https://someonewhocares.org/hosts/ipv6zero/hosts/ -o /etc/hosts

0.0.0.0 is NXDOMAIN (non-existent domain) - this will sinkhole requests
Some lists use 127.0.0.1 (localhost) which exists, and will probably not respond, adding delay
Remove 'ipv6' from someonewhocares.org/ URL for an IPv4 only version

Flush the DNS cache to activate the new hosts file:

sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

Once installed, you'll find that your surfing experience improves, your breath becomes more lovely, and the world transforms into a gentle place full of unicorns and love. If this is your thing, check back with someonewhocares regularly for their latest download.

Stealth Mode

OS X has a built-in Firewall, and one of the options is Stealth Mode. If this option is checked, the Mac will not respond to PING's.

To allow a network PING response (including localhost):

Open: System Preferences ➤ Security & Privacy ➤ Firewall ➤ Advanced.
You may have to authenticate (click the padlock) with your Admin password to enter the Advanced features.
Uncheck Enable stealth mode.

Don't forget to re-enable stealth mode when you connect to the internet.