You already use this technology every day
Bonjour is Apple's zero-configuration networking protocol. It has been built into every Apple device for over two decades. It is the technology behind AirDrop finding nearby devices, AirPlay discovering your Apple TV, your Mac automatically finding a network printer, and your iPhone seeing a nearby HomePod.
Every time you see a device appear in a list without you having to type an IP address or go into settings — that is almost certainly Bonjour doing the work.
Our apps do exactly the same thing as AirPlay — except instead of discovering a speaker, your iPhone discovers your library.
What actually happens when you open the app
When the app is running on your Mac and sharing is enabled, it broadcasts a Bonjour announcement on the local network. Think of it as the Mac quietly saying "I'm here, and I have a library" — once per second, to anyone listening on the same network.
When you open the app on your iPhone or iPad, it starts listening for those Bonjour announcements. Within a second or two, it hears the Mac — and immediately knows its name, its address on the local network, and the port to connect to. No typing required.
Your iPhone connects directly to your Mac over your home Wi-Fi. Not through Apple's servers. Not through any relay. A straight line from your phone to your Mac, entirely within your home network.
Your photos or music — filters, albums, artwork, metadata — all served directly from the Mac. The iPhone renders them natively. The whole exchange happens over Wi-Fi at whatever speed your router supports, typically fast enough to load a full-resolution photo or start a track in under a second.
Multiple Macs, one library
If you have your library split across two Macs — say, a desktop with the main archive and a laptop with recent additions — both can run the app simultaneously. Your iPhone hears both Bonjour announcements, connects to both, and presents the combined contents as a single unified library, with deduplication handled automatically.
There is no merging, no copying, no syncing between the Macs. Each Mac serves its own files. Your iPhone simply asks both of them and assembles the result. From your perspective, it is one library.
It is like having two bookshelves in different rooms. You do not need to move all the books to one shelf to read them — you just know where both shelves are and can pull from either.
Why it stops at your router
Bonjour announcements are local network broadcasts. They travel over your Wi-Fi but stop at your router — they do not go out to the internet. A device outside your home, even one that knows your IP address, cannot receive these announcements and cannot initiate a connection to your library.
This is not a firewall rule you have to configure. It is simply how local network broadcasts work. The announcements are addressed to every device on your network, but "your network" ends at the router.
The internet cannot see your library because the library never announces itself to the internet. It only speaks to devices that are already inside your home network — and even then, only to devices that are actively listening for it.
There is no open port, no forwarded address, no way to reach it from outside. It is not that the door is locked. There is simply no door facing outward.
What happens when you leave the house
When your iPhone leaves your home network — the moment you step outside and your phone switches from Wi-Fi to cellular — it can no longer hear the Bonjour announcements from your Mac. The library becomes unreachable, and the app will show no connected library.
This is intentional. Remote access would require either a relay server (which we deliberately do not have) or opening a port on your router (which we deliberately do not ask you to do). The tradeoff is that your library is available everywhere in the house and nowhere outside it.
A library that is only accessible inside your home is a library that cannot be accessed by anyone outside your home. The limitation and the security guarantee are the same thing.