Next, let's examine the IGMPv1 membership report.
The source address of the membership report is the
client that is requesting multicast.
In this case, the Globomantics client.
The destination is the group address that the client wants to join.
A quick look at the Wireshark capture summary shows two
membership reports for two separate groups,
one for 239.2 .2 .2, and one for 239.1 .1 .1.
The type of the message is 12 in hex, or 18 in decimal.
This type is specific to IGMPv1 and is not recycled in later versions of IGMP,
and it is also different than the query message.
As I discussed in my ICMP Protocol Deep Dive course,
using different packet types is a simple way for protocol
designers to differentiate between messages,
which can streamline how they are processed by network devices.
The critical piece of information is the multicast
group the client wants to join.
In this case,
the client has answered the query with I want multicast on group 239.2 .2 .2.
The IGMP membership report carries exactly one multicast group.
This explains why the client joined multiple groups by
sending multiple membership reports.
A client joining hundreds of groups, such as video collection server,
could create quite a lot of IGMP chatter on the network using IGMPv1.
I'll quickly show the second IGMPv1 membership report to illustrate this point.
This membership report is nearly identical to the one we viewed previously,
except with a significant change of joining another multicast group.
Here the client joins 239.1 .1 .1,
indicating its interest in receiving multicast sent to this group within the network.