7.6 AirPrint Deployment Considerations in iPrint Appliance

The mDNS protocol operates on service announcements and queries, which allow devices to advertise specific applications, such as printing services. Each query or advertisement is sent to an mDNS multicast address for delivery to all clients on the subnet. Apple's Bonjour protocol relies on Multicast DNS (mDNS) operating at UDP port 5353. iOS devices listen for services on these reserved group addresses:

The addresses used by the mDNS protocol are link-local multicast addresses and are valid on the link local domain. Hence, routers might have to be enabled, in order to forward the multicast advertisement to various subnetworks.