Comment 13 for bug 44062

From http://o.bulport.com/index.php?item=55:

Cookies with "indirectly" illegal domains

It is a bit complicated with unregistered domains such as "specialized" national
ones co.uk, co.jp. How can Opera know if yy.zz is a "specialized" national
domain, suffix for many other registered domains, or is itself an usual
registered domain in national zz domain?

The answer is simple. Opera can use Domain Name Service to check if yy.zz is a
registered domain. If the check fails, Opera assumes yy.zz is "specialized"
national domain.

Thus if site D (www.domD.yy.zz) wants to set a cookie, ordering it to be
accessible to yy.zz, Opera will first check (using Domain Name Service, DNS) if
yy.zz can be contacted on the Internet. If DNS check fails, Opera will accept
the cookie, but will silently restrict the later access to the cookie just to
the site D's server www.domD.yy.zz, instead of allowing it to all servers in the
yy.zz domain.