Here are some brief answers to your questions..
a) Since the concentrator is not an inline device, regular traffic will flow in and out just fine when you put it in (it has one public interface and one private interface).
b) The concentrator translates incoming VPNs to their assigned VPN pool address. Then it gives access based on whatever networks are defined to go down the tunnel.
c) The concentrator is NOT inline, but it will need its own public IP.
D) The concentrator will NOT provide firewall abilities. However, if you buy an ASA, it works as a concentrator (not as good as a dedicated concentrator) and a firewall. I would recommend the ASA if you are more concerned with inbound/outbound access or if you don't already have a firewall. The VPN concentratores are solely for VPNs.
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Thanks.