13.2 IGMP Message

13.2 IGMP Message

Figure 13.2 shows the format of the 8-byte IGMP message.

Figure 13.2. Format of fields in IGMP message.

The IGMP version is 1. An IGMP type of 1 is a query sent by a multicast router, and 2 is a response sent by a host. The checksum is calculated in the same manner as the ICMP checksum.

The group address is a class D IP address. In a query the group address is set to 0, and in a report it contains the group address being reported . We'll say more about it in the next section when we see how IGMP operates.