Magento 1.9 Why doesn't admin login work if Cookie Domain is set?

Question

Magento

Help me to understand.
If you set Cookie Domain - NULL , then everything is OK, the login to the admin panel works.
But then the login for users does not work, because. two "frontend" cookies are set.
If you set Cookie Domain - www.mydomain.com , the situation is reversed.
For users it works, there is no admin panel.
Or how to disable the redirect from www.mydomain.com to domain.com when entering the admin panel?

Answer 1

Leave the field blank and it will be the default.
Moreover, if you have subdomains, then they will also be distributed as it is installed on .mydomain.com