A unique value for each interface. Its value ranges
between 1000 and the value of ifNumber. The following
information is encoded in the ifIndex:
a. An ifIndex between 1000 and 9999 define
hardware interfaces with the following attributes.
b. An ifIndex greater than or equal to 10000 defines
a software interface. Examples include the
dialup-interfaces as defined in the biboPPPTable.
The interface's address at the protocol layer
immediately `below' the network layer in the
protocol stack, as long as it differs from the factory
setting which is indicated in field OrigPhysAddr.
For interfaces which do not have such an address (e.g.,
a serial line), this object should contain an octet
string of zero length.
Default value is the empty string.