Identity of address attempting connection

I have Norton Internet Security for Mac 5.7 (running under Yosemite). When I open my Mac in the last few days I regularly get a message warning me of a connection attempt. The remote address attempting to connect is: 224.0.0.251. To be on the safe side, when asked what to do I picked "block", BUT I have no idea what this is, i.e. wether t is legitimate connection attempt or not. How do I find out what this 224.0.0.251 remote address corresponds to? For instance, could it be Apple TV connecting through AirPlay? or ...???