Redirection Error

i just got code for .htaccess to redirect mydomain.com to a directory on the same domain here directory123 is that name of directory where i wanna redirect my domain but problem is that when i put complete url like
www.mydomain.com then it will successfull redirect to www.mydomain.com/direcotry123

But when i put address like mydomain.com without www then it will not redirect kindly check and let me know the correction to redirect it as well

Code:
# -FrontPage-

IndexIgnore .htaccess */.??* *~ *# */HEADER* */README* */_vti*

<Limit GET POST>
order deny,allow
deny from all
allow from all
</Limit>
<Limit PUT DELETE>
order deny,allow
deny from all
</Limit>
AuthName mydomain.com
AuthUserFile /home/username/public_html/_vti_pvt/service.pwd
AuthGroupFile /home/username/public_html/_vti_pvt/service.grp

#RedirectMatch permanent ^/$ [url]

RewriteEngine on
RewriteCond %{HTTP_HOST} ^www.mydomain.com$ [OR]
RewriteCond %{HTTP_HOST} ^www.mydomain.com$ [NC]
RewriteRule .* http://www.mydomain.com/directory123