Dear Sir,

Can anyone help me on this topic.

1. How secure is Internet Authentication Service (IAS) that support
Remote Authentication Dial-In User Service(RADIUS)?

2. Advantage & disadvantage of IAS&RADIUS compare with DMZ?

I really appreaciate your help and time
Thank you so much

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Secure against what threat? IAS is as secure as any other RADIUS
implementation

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I am not sure that these two things can be directly compared:
- RADIUS is an authentication/authorization mechanism (typically used for
VPNs or dial-up clients)
- a DMZ is a way of isolating one portion of a network (internet facing)
from another (your private, internal, network).

Cheers
Ken