asterisk users - Dec 2009 - digest authentication method and the realm domain

Hi All;

When using the digest authentication method, so I have to create the realm
domain with its username and passwords to be used for SIP digest authentication,
correct?

Now, how to create this domain? Should be reachable (can be ping) from a remote
device?

In other words, to create this realm domain, do I need to do a configuration on
the linux, or only this to be done at the Asterisk configuration files, so I can
reach this domain from the remote machines?

I am really in need for a help in this topic, as I did not find enough
information.

Any help?

Regards
Bilal Ghayad