In IPv4, 0.0.0.0 is a special address, aka INADDR_ANY
, that means "bind every possible address on every interface".
So, the multicast network at 224.168.2.9, if it's reachable at all, will certainly be reachable from a socket bound to 0.0.0.0.
Meanwhile, 127.0.0.1 is a special address, aka INADDR_LOOPBACK
, that means "bind localhost only on the loopback device". There's no way to reach anything but the local host itself on that socket. In particular, you can't reach your multicast network. Whether you get an ENETUNREACH
, ENETDOWN
, or EADDRNOTAVAIL
is platform-specific, but whether it works is not—it can't possibly work.
If you want to test multicasting without testing across multiple computers, you will need to set up a loopback network with more than one address, so you can bind the client, the server, and the multicast group all to different addresses within that network.