[merb]

multicast is like a broadcast except that it is authenticated, i.e each device that is authenticated against a certain multicast will get the content.

it's mostly used for internet television or other multimedia stuff.

(some stuff is/was unallocated, since some early users tought it's a good idea to actually use some unallocated stuff to do bgp..., testing or routing per se (especially cisco routers) or even login pages, exist nodes, i.e. 1.0.0.0 was a problematic ip, but since cloudflare grabbed the 1.1.1.1 I think people will stop doing stupid things)

[sofaofthedamned]

It's nothing to do with authentication, it's simply indicating you want the data. If authentication or encryption is used it's nothing to do with multicast.