RewriteCond %{REQUEST_URI} !^/mailman/.* 
RewriteCond %{REQUEST_URI} !^/pipermail/.* 
RewriteCond %{REQUEST_URI} !^/cgi-bin/.* 
RewriteCond %{REQUEST_URI} !^/icons/.* 
RewriteCond %{REQUEST_URI} !^/cacti/.* 
RewriteRule ^(.+) %{HTTP_HOST}$1 [C] 
RewriteRule ^([^.]+)\.([^/]+)\/(.*)$ /var/www/web/$2/$1/$3 [L] 

Which maps http://one.two.three/four to /var/www/web/two.three/one/four