A unique value for each interface of this physical port.
It is a value between 1 and the number of interfaces that
exist for the current port. The number of interfaces for
each type of port is: 24 for a port type (wanPhysPortType)
of 't1', 30 for a port type of 'e1', and 1 for a port type
of 'synchronous'. There will be no entries if the port type
is 'none'.