connect to AD from different environment

i have a certain service that gets/needs to connect to AD to get the users that can access to the services. This services is on a different machine, outside the domain of AD (different ISP).
this means that AD needs to be availible for the outside world. What kind of protection can i use to make this a secure process? is ssl prossible? other ideas?
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Glen KnightCommented:
What sort of query does it use?  Bear in mind that even a simple LDAP query means your Active Directory data is available outside of your network.

Create ADAM instance in your dmz, then sync data required for external services with your AD using ADAMSync. This way only ADAM instance with limited data will be exposed to external world.

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Is this a web service? I would think, would need to have a route the 'other' network address. The 'other' network  router would in turn (the one offering the service) have a rule allowing your network subnet access to their specific public IP and their specific ports.
You might need a DNS entry (zone file) for the 'other' network or at the very entry in each of your machines hosts' file.
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