External vs. internal
When you consider what the CAS role does, it makes perfect sense to distinguish between two sources of client connections: internal clients on the organization’s network and external clients that connect through a firewall by using one of the supported and enabled client protocols. Some organizations want to allow both internal and external clients to connect without restriction, whereas others limit external connections. Most organizations that deploy Exchange also want the flexibility to configure CAS behavior independently for internal and external connections.
The primary objects of interest for the external-versus-internal split are the URLs to which clients connect and the authentication methods that clients may use. ...
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