It's possible to make a cookie valid for all subdomains by adding a specific domain property like google.com.

If we set the domain property of a cookie to facebook.com, for which of these links will the cookie be valid?

blog.facebook.com ads.facebook.com about.facebook.com 

If we set the domain property, then the cookie will be valid for all subdomains.

Since the domain property of this cookie is set to facebook.com it will be valid for all subdomains.