Google provides unique, domain-dependent OpenIDs. E.g.
two.domain.com will not have the same unique identifier. How can different Stack Exchange websites map my Google OpenID across each other?
I would like to implement a similar functionality for my websites. (E.g. same Google user across all websites — Google does not provide the same identifier across domains for security reasons, so it is not possible the easy way — how is this possible?)
What other famous providers use different unique identifiers across domains instead of the same unique ID?