Mike Albano writes on his dumping ground :

Long story short: Bonjour service advertisements, including those sent from Apple TV’s, have a destination address of link-local multicast 224.0.0.251. What this means is that the advertisements,by spec, will not cross a subnet boundary.
Who cares if at home & kind of a nightmare in the Enterprise.

Read more at: Using mDNS Snooping for ‘Bonjour’ support