Sometimes, Google or other search engines start to crawl your subdomains, indexing everything without your knowledge. And your search results start to appear something like this – http://subdomain.mainsite.com/page.html, instead of http://mainsite.com/page.html. All paths to pages are same except the subdomain added to the URL.
To redirect all your subdomains to main site URL, add following line in your .htaccess file.
[cc lang=”htaccess”]
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www.)?[^.]+.mymainsitename.com.*$
RewriteRule (.*) http://mymainsitename.com/$1 [L]
[/cc]