Step 3 – Edit the hosts file

The hosts file contains some comments (lines starting with the # symbol), as well as some default hostname mappings (e.g. 127.0.0.1 – localhost).
Simply append your new mappings underneath the default ones. Or edit one of the default values if you know what you are doing!
You can navigate the file using the arrow keys.

Example HOSTS File entry:

Syntax: HOST IP DomainName DomainName DomainName ....

Each subdomain like mail.Mydomain.com, something.mydomain.com has to be entered seperately to the HOSTS file.

72.249.5.10 mydomain.com www.mydomain.com

The above entry will force your PC's DNS for MyDomain.com to 72.249.5.10 instead of the live site.

Once you don't need the entry, you can either comment it out or delete it.  Commented out entry will look like this:

#72.249.5.10  mydomain.com www.mydomain.com

# sign comments out an entry in the HOSTS file.

Step 4 – Save the hosts file

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

Step 5 – Flush the DNS cache

On Leopard you can issue a simple Terminal command to flush the DNS cache, and have your host file changes to take immediate effect:

dscacheutil -flushcache

All Steps Complete: You can now test your new mapping on the browser!  You may need to restart your browser in case your browser uses DNS pre-fetching.