224.0.0.0/4

Multicast addresses are used for one-to-many delivery. A sender sends traffic to a multicast group address, and hosts that joined that group can receive it.

This block is not publicly routable as ordinary unicast Internet traffic. Multicast forwarding requires explicit network support and scope controls; most multicast a beginner system administrator sees is local to a LAN (local area network) or an organization's own routed network.

A system administrator may encounter multicast in routing protocols, service discovery, IPTV (Internet Protocol television), imaging systems, clustering, or monitoring tools. Addresses near 224.0.0.x are especially common for local network control traffic and should not be expected to cross routers.

Firewall tip: multicast is not a valid ordinary source address, and it is normally not allowed as an Internet destination unless you intentionally run multicast routing. Edge firewalls commonly put 224.0.0.0/4 in both "bad source" and "bad destination" filtering rules.