Why is dot needed to make a cookie available among subdomains?

We need to set the domain as .example.com to make it available for *.example.com,

but why not 开发者_运维知识库example.com?

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example.com means "exactly" example.com ; and nothing more.

If example.com was used to mean *.example.com, how would you set a cookie that's valid on example.com and not on subsite.example.com ?

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The dot at the beginning says it'll be available for all subdomains. It's just... needed :)

If you don't use the initial dot, you'd be telling it to be only available for example.com (not even www.example.com).