Airplay not connecting w/Denon AVR-1912
I hope this helps people that find themselves in the same extremely frustrating situation that I found myself in over the past few days!
This is an answer to a question that is repeatedly asked, namely; why can't iTunes find your Denon AVR-1912?
I found this solution on a Verizon support board and it worked successfully.
I have a Denon AVR-1912 w/Airplay and a MacBook running ITunes 10.6.3 on a Verizon Fios network with an Actiontec wireless modem. This network also supports 2 iPhones, an iPod Touch, an AppleTV and 2 Roku units. Suddenly, for reasons that remain inexplicable, all Apple products stopped giving the option for the Denon receiver on the Airplay icon in iTunes. No software or firmware updates preceded this... it just happened, because god hates man I suppose..
I rebooted, reset, unplugged, updated... to no avail. I then found this nice solution:
(Go to your modem's browser/IP and go to the "Advanced" link)
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OK I have this working reliably, now. AND with still being able to hit outside my LAN 
If you go to Advanced (Accept the warning) and go to the IGMP Proxy menu and disable IGMP Proxy.
This magically makes it work! I still haven't read up on what the IGMP setting does, but it is a proxy, which obviously interfereres in this case with AIRPLAY udp messages being broadcast and received. Hopefully it doesn't adversely affect other devices on my network, but I can cross that bridge then.
