Options +FollowSymlinks
RewriteEngine on
RewriteCond %{REMOTE_HOST} !^123\.45\.67\.89
RewriteCond %{REQUEST_URI} !/alternate_page\.html$
RewriteRule \.html$ /alternate_page.html [R=302,L]

http://www.webmasterworld.com/forum92/167.htm

This code redirects all external traffic (all traffic not belonging to a particular ip) to a temp page. This is useful for performing site maintenance.

Redirect all external traffic to a temp page

redirect, htaccess, ip

Apache