The network address which, when combined with the
corresponding value of hwIpMcastRouteSourcePrefixLength,
identifies the sources for which this entry contains
multicast routing information.
This address object is only significant up to
hwIpMcastRouteGroupPrefixLength bits. The remainder of the
address bits are zero. This is especially important for
this index field, which is part of the index of this entry.
Any non-zero bits would signify an entirely different
entry.
For addresses of type ipv4z or ipv6z, the appended zone
index is significant even though it lies beyond the prefix
length. The use of these address types indicate that this
source address applies only within the given zone. Zone
index zero is not valid in this table.