share session accross sub domain with different ip-address

[eluser]phpmonster[/eluser]
Not sure why the hosting company decided to assign a separate IP address for each sub-domain but thats causing some problems on sharing session data.

How do I share session data to my sub-domain which has a different ip than the main domain?

currently have:

$config['cookie_prefix'] = '';
$config['cookie_domain'] = '.mydomain.com';
$config['cookie_path'] = '/';