The hosts file in a computer represents a simple plain text file, where are mapped hostnames to IP addresses (wiki).

To edit this file on Mac OS X is a very simple task:

• Open the Terminal.app 
  (using Spotlight : command + space  – and then write terminal)
• Edit the file /private/etc/hosts
  (you can use vim or nano …. ex: sudo nano /private/etc/hosts)
•  Add the new map:
  add at the end the new map you want : ex: 127.0.0.1         new.address.com
• Save the hosts file
  save the file with the new map

If you want that your change to take place immediately, run the following command:

• dscacheutil -flushcache

Now you can test the new mapping.