Description: The Circuit Number that this interface runs over. If Circuit Number is between 1 and 1023, it identifies a subnetwork/datalink layer interface, i.e. the layer below the network layer. A subnetwork layer interface is associated with a physical link or, in some rare cases, with a group of physical links. Generally, more than one subnetwork layer interface can be defined on a physical link (e.g. channels on a T1 link).

If the Circuit Number is higher that 1023, it's a circuitless interface, i.e. a logical IPv6 interface that is not associated with any subnetwork layer or physical layer interface.

Zero indicates that it's a tunnel endpoint, i.e. IPv6 packets are encapsulated and transmitted by another network layer protocol or another instance of the IPv6 protocol.