A unique value for each MAC on the managed
entity. The MAC identified by a particular value
of this index is that identified by the same value
of an ifIndex object instance. That is, if a MAC
is associated with the interface whose value of
ifIndex in the Internet-Standard MIB is equal to
5, then the value of snmpFddiMACIndex shall also
equal 5. The value for each MAC must remain
constant at least from one re-initialization of
the entity's network management system to the next
re-initialization.