With a WordPress Multisite Network messing with the .htaccess can be slightly disconcerting. However, if you’ve got both the .com and .net versions of your domain, visiting the version of your domain that doesn’t have your network installation will take you to the registration page of your multisite network.

The good news: adding this code to the top of your .htaccess both doesn’t break your network, and does redirect everything properly:

Make sure to change ‘url’ in this example to the domain you are redirecting.
RewriteCond %{http_host} !^url.com$ [NC]
RewriteRule ^(.*)$ http://url.com/$1 [R=301,NC,QSA,L]