I have a Dual-Band N (2.4 & 5 GHz) router (Linksys E3000). My mac and other computers in my house connect to the 5 GHz and my iPhone (4S) connects to the 2.4 (because it cannot connect to a 5 GHz network).
I have an Airport Express (with speakers connected) that is connected to the 2.4 GHz network so that I can play songs from my iPhone. I thought that I will not be able to play songs from iTunes on my Mac. To my surprise iTunes is able to discover the speakers on the Airport even though the mac is not connected to the 2.4Ghz network at all. (Confirmed this from iTunes on a PC as well)
Does iTunes establish its own connection? Is there any other trick involved?