CodeIgniter session cookie domain

Question

It appears with the following cookie settings below, that cookies are set for the entire domain. For example if I was on http://subdomain.domain.com the cookie would be set for subdomain.domain.com if I was on domain.com the cookie would be set for domain.com.

If the cookie is set for subdomain.domain.com that does that mean subdomain2.domain.com cannot access a cookie from subdomain.domain.com?

/*
|--------------------------------------------------------------------------
| Cookie Related Variables
|--------------------------------------------------------------------------
|
| 'cookie_prefix' = Set a prefix if you need to avoid collisions
| 'cookie_domain' = Set to .your-domain.com for site-wide cookies
| 'cookie_path'   =  Typically will be a forward slash
|
*/
$config['cookie_prefix']    = "";
$config['cookie_domain']    = "";
$config['cookie_path']      = "/";

Answer 1

When using several codeigniter applications on subdomains it is also important to set the same $config[‘encryption_key’].

Answer 2

Domain specific cookies are only accesible on that domain. Use ".domain.com" as your cookie domain to be able to use it across all subdomains and main domain.