There are 3 different ways of doing this:

a.  URL forwarding. Can be done through your domain name control panel at most registrars, or your hosting name servers. The forwarded address is displayed in your browsers address panel.

b.  Framing. Leaves the original name (alias) in your browsers address panel. Not too wonderful cause if you submit this address to search engines, their spiders won't get to your sites content.

c.  CNAME (Alias) forwarding. Very few domain name registrars can provide this. For global domains try registerfly.com and UK domains just-the-name.co.uk.  CNAME forwarding allows the new URL to be displayed in the browsers address panel, and duplicates file names too. Search engine spiders can access the site content. Problems will be encounted if  you're intending using forms on your web pages.

i).
For setup at Registerfly, refer to their instructions. Must point to an IP address. Following entries ensure both with and without www. prefix.

www               CNAME (Alias)                yourdomain.com.
@                     ADDRESS                        your IP address   

(Note: the entry for yourdomain.com on the first line, must end with a period).


ii).
For setup at Just-the-Name.co.uk you may add upto 10 sub domains, however all including index must point to an IP address.