Although there is the RFC 2965 (Set-Cookie2, had already obsoleted RFC 2109) that should define the cookie nowadays, most browsers don’t fully support that but just comply to the original specification by Netscape.
There is a distinction between the Domain attribute value and the effective domain: the former is taken from the Set-Cookie header field and the latter is the interpretation of that attribute value. According to the RFC 2965, the following should apply:
- If the Set-Cookie header field does not have a Domain attribute, the effective domain is the domain of the request.
- If there is a Domain attribute present, its value will be used as effective domain (if the value does not start with a
.it will be added by the client).
Having the effective domain it must also domain-match the current requested domain for being set; otherwise the cookie will be revised. The same rule applies for choosing the cookies to be sent in a request.
Mapping this knowledge onto your questions, the following should apply:
- Cookie with
Domain=.example.comwill be available for www.example.com - Cookie with
Domain=.example.comwill be available for example.com - Cookie with
Domain=example.comwill be converted to.example.comand thus will also be available for www.example.com - Cookie with
Domain=example.comwill not be available for anotherexample.com - www.example.com will be able to set cookie for example.com
- www.example.com will not be able to set cookie for www2.example.com
- www.example.com will not be able to set cookie for .com
And to set and read a cookie for/by www.example.com and example.com, set it for .www.example.com and .example.com respectively. But the first (.www.example.com) will only be accessible for other domains below that domain (e.g. foo.www.example.com or bar.www.example.com) where .example.com can also be accessed by any other domain below example.com (e.g. foo.example.com or bar.example.com).