Multicast DNS (mDNS) is a way of using familiar Domain Name System (DNS) programming interfaces, packet formats and operating, without configuring a conventional DNS server. It is useful in small networks without a DNS server, but can also work in environments beside a DNS server. mDNS functionality is provided using IP multicast over User Datagram Protocol (UDP).
The mDNS protocol is used by Apple's Bonjour, GNU/Linux Avahi, and Oracle Grid Naming Service (GNS) service discovery systems.
Read more about Multicast DNS: Packet Structure