Every domain will be acessible by
http://www.example.com and http://example.com
as
http://www.subdomain.example.com and http://subdomain.example.com
Including any subdomain, to solve this problem you just need do a 301 redirect to force your visitors to acess only. This will be good for your SEO and google ranking because your page rank is shared by prefix www. and non-www
For example:
10 strongly ranked sites links the page: www.example.com/page.html
5 strongly ranked sites links the page: example.com/page.html
Your pagerank points will be shared to 2 "differents" websites
How to solve, the best simple way:
Create a .htacess (or edit if you already have)
RewriteEngine on
RewriteCond %{HTTP_HOST} ^www\. [nocase]
RewriteRule .* http://eg.example.com%{REQUEST_URI}%{QUERY_STRING} [redirect=301,L]