Naturally,if a computer wants to join a group, it does not have to wait for a membership query; it can inform the router immediately about the group it wants to join.
Section 17.3.3 describes how exactly the IGMP protocol works.
In addition to the tasks discussed above,IGMP is used for other things.
The following list summarizes everything the IGMP is used for:
Query a multicast router for groups desired in a LAN.
Join and leave a multicast group.