Simple definition
mDNS (Multicast DNS) resolves names locally on a network without querying a central DNS server.
Technical definition
Defined by RFC 6762, mDNS uses UDP port 5353 and multicast address 224.0.0.251 for IPv4 or FF02::FB for IPv6. It is commonly used for names in the local ‘.local’ domain. Queries and responses are link-local, so they do not naturally cross routers or VLAN boundaries without an mDNS reflector, relay, or gateway.
What is it used for?
Automatically discover and resolve local devices and services such as printers, Apple devices, Chromecast endpoints, or services advertised through DNS-SD.
Practical example
A computer looks up printer.local. It sends an mDNS multicast query on the LAN and the device replies with its address without requiring a conventional DNS record.
Common issues
- Multicast blocked by Wi-Fi, switching, or firewall configuration
- Discovery fails across VLANs because no mDNS relay is present
- Conflicting .local names
- Excessive multicast traffic on a poorly segmented network
Key takeaway: mDNS is designed for the local link; discovery across VLANs requires an explicit relay or gateway design.