This option is enabled by default. In other words, by default you will get a copy of all multicast packets you send, from each of the interfaces that you've joined as a group member. You need to use this option if you want to send multicast packets beyond the local network, since the default TTL for multicast packets is one. You can use any valid host group address as a destination address in an expanding ring search. Multicast Mechanics You don't need to know a lot about the mechanics of multicast support since the WinSock API insulates you from the low-level details.
Every host remains a member of the all-hosts group for as long as the host is active.
If it doesn't receive any reports for a particular group after some number of queries, then the routers assume that that group has no local members and that they need not forward remotely-originated multicasts for that group onto the local network. Instead it starts a random delay timer for each of its group memberships on the network interface of the incoming query. As a result, each host membership query typically generates only one report for each group present on a network. A multicast host sends a host membership report for the all-hosts group only when it initializes each multicast interface at boot time.