In my opinion, the best example of transparent firewalling in the way described is vlan pairs on the IPS appliance. The concept of using transparent Firewall to bridge two VLANs is exactly the same as using a VLAN pair on an IPS to bridge two vlans. In both cases, both of the vlans exist in the same layer 3 subnet. However, for devices in vlan x to talk to devices in vlan y, they must go through the transparent firewall, just like they would have to go through the IPS. The IPS simply has a less strictly defined role than that of a firewall.
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