The interface group ID. There are two parts for this id
depicted as follows:
31 7 0
+++++++++++++++++++++++++++++++++++++++++
+ entity index + group index +
+ ( bit 8..31 ) ( bit 0..7 ) +
+++++++++++++++++++++++++++++++++++++++++
From bit0 to bit7 (one byte), it is interfaces group index.
From bit8 to bit31 (three bytes), it is the physical index
the same as the entPhysicalIndex specified in ENTITY-MIB.
Note that only entPhysicalIndex values that represent
physical boards (i.e., associated entPhysicalClass value is
'module(9)') are permitted to exist in this table.