It may depend on your OS. This module creates a socket of type SOCK_DGRAM. These route at the IP/UDP level. I know the module says it can limit you to what's on one NIC, but I'm pretty sure on Linux there's no way to enforce this. The point of multicast is to deliver to every endpoint using that destination address. Maybe the Windows stack has hooks to limit this. I am curious as to why you need to listen for the mcast traffic on a socket if you're using tcpdump to see them as well. You can pass a filter to tcpdump to only see what you're looking for.