The IfIndex value for the interface to which this boundary applies. Packets with a destination address in the associated address/mask range will not be forwarded over this interface. For IPv4, zone boundaries cut through links. Therefore, this is an external interface. This may be either a physical or virtual interface (tunnel, encapsulation, and so forth.) For IPv6, zone boundaries cut through nodes. Therefore, this is a virtual interface within the node. This is not an external interface, either real or virtual. Packets crossing this interface neither arrive at nor leave the node, but only move between zones within the node.