This object holds the ifIndex of the port, channel, subchannel, or virtual circuit that an IP interface is bound to, when that underlying layer is represented in the standard IF-MIB; it's set to zero if no such direct binding exists. An administrator uses this to trace which physical or logical interface layer an IP binding is actually riding on, useful when correlating IP-layer configuration with interface-layer statistics. A nonzero value here directly identifies the corresponding ifTable row for that IP binding's underlying transport.