17.3.1 Formatting and Transporting IGMP Packets
IGMP messages are transported in the payload field of IP packets,and the number 2 in the Protocol field of the IP packet header identifies them as IGMP messages.
They are always sent with the TTL value one,which means that they cannot leave the area of a subnetwork and so means that IGMP manages group memberships only within a subnetwork.
To distribute this information beyond these limits, we have to use multicast routing protocols.