The software that comes with the iPod photo disables a RealNetworks workaround that made it possible to play music downloaded from Real's online music store on the iPod.
According to RealNetworks, other iPods are still able to play Harmony downloads from Real's store.
Real discovered a way of making it possible to play music from its store on the iPod about four months ago, sparking a row with Apple, which claimed that Real was using the "tactics of a hacker".
At the time Apple warned iPod users not to download music from Real's store, as the songs would "cease to work with current and future iPods."
According to a CNet report, some figures in the music industry took the contrary view however, applauding RealNetworks' attempt to create compatibility between its store and the iPod.
Real has promised to look for ways to get around Apple's blocking of music files from its competing online music store. RealNetworks Matt Graves told The Mac Observer: "We are currently evaluating our options on providing iPod photo users with the ability to play music files from our online music store.
"We remain fully committed to providing consumers with the freedom to use the music libraries they purchase from us on different portable audio devices, both now and in the future – including the iPod photo."