RewriteEngine on
RewriteBase /
RewriteRule ^/$ /default.htm [R]

Url tried -> http://localhost/

127.0.0.1 - - [01/Aug/2006:10:45:53 +0200] [localhost/sid#624560][rid#6f4bc0/initial] (3) [per-dir /var/www/] strip per-dir prefix: /var/www/ ->
127.0.0.1 - - [01/Aug/2006:10:45:53 +0200] [localhost/sid#624560][rid#6f4bc0/initial] (3) [per-dir /var/www/] applying pattern '^/$' to uri ''
127.0.0.1 - - [01/Aug/2006:10:45:53 +0200] [localhost/sid#624560][rid#6f4bc0/initial] (1) [per-dir /var/www/] pass through /var/www/