Apple's Bonjour protocol can't communicate across networks on its own, but vendors are responding with gateways that let Bonjour find resources on other subnets.
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Apple Bonjour is a service discovery protocol that identifies resources such as printers and servers on a LAN without needing to know IP addresses or deal with DNS entries. Bonjour provides a simple naming and addressing mechanism that adds to the ease of use for which Macs are justifiably famous. With the rise of the iPad and iPhone, Bonjour enhances its value as a core element of iOS. Capabilities such as AirPrint and AirPlay, both of which are dependent on Bonjour, offer convenience that many iOS users find indispensable. In business settings, however, Bonjour has a key limitation: It's a Layer-2 -- not a Layer-3 -- protocol.
What this means is that the cross-IP-domain interoperability and flexibility that we often take for granted isn’t available with Apple Bonjour; it's not a routable protocol. When it comes to Bonjour, what happens on a subnet stays on that subnet. Moreover, Bonjour often generates significant amounts of multicast traffic, which can really burden an enterprise LAN.
Bonjour, meet the enterprise
For this reason, several networking suppliers have introduced Bonjour gateway products that address both of these issues. These gateways make Bonjour routable without requiring that clients know anything about addressing, naming or location. The Bonjour gateways also convert multicast to unicast, limiting the amount of multicast traffic.
Apple Bonjour is an implementation of Zero Configuration Networking (zeroconf), a standard of the Internet Engineering Task Force (IETF), which also applies to Bonjour competitors such as Microsoft’s Universal Plug and Play. Zeroconf is valuable in smoothing and simplifying network operations whether Bonjour is required or not. Onboarding is eased thanks to IP address auto-configuration, which, depending on the implementation, can work with both IPv4 and IPv6. Naming is implemented via the Multicast Domain Name Service, which also aids in service discovery. Apple has been a leader in the development of the IETF standards here, and has contributed a significant amount of technology to the effort. As a result, Bonjour is widely respected and consequently widely used, with implementations available even for Windows.
In operation, Apple Bonjour literally fades into the woodwork (OK, network) and operator intervention is very rarely required. With today's Bonjour gateways, that transparency is now available across very large networks. It's a safe bet that almost every Apple user has been a Bonjour user without even knowing it. Given the complexity for which networking is famous, what could be better than that?