A numerical distinguisher relevant for the loopback and null IP
interface types.
loopback Must be unique for all IP interfaces of this type.
null Must be unique for all IP interfaces of this type.
(FOR IMPLEMENTATIONS THAT SUPPORT ONE AND ONLY ONE NULL
INTERFACE, THE VALUE ZERO MUST BE USED.)
For all other IP interface types, this object is not relevant, reports a
value of zero when read, and must be given a value of zero if explicitly
configured during creation.