What is 224.0.0.251?

So I don't normally use my Wifi on my computer, but I recently moved and my new place uses only Wifi. I was using the internet, and when I turned off my Wifi (I like to disconnect from the internet before putting my computer to sleep or turning it off, just in case), I got a bunch of weird console messages I've never seen before. I Googled it but didn't come up with anything particularly informative. Here's the message, maybe you guys can puzzle it out:

mDNSResponder[21] mDNSPlatformSendUDP sendto failed to send packet on InterfaceID 00810400 en1/4 to 224.0.0.251:5353 skt 7 error -1 errno 50 (Network is down) 1814560111

This message appeared 7 times, the only difference each time being that long number at the end. Anyone know what this means? I've narrowed out a couple of things:

1) Since a Google search turned up results, it's not my router or any other computer on my network.

2) Google turned up entries from a Debian mailing list, so I don't think this is Mac OS X specific.

Thanks.